Add opt-in theme system + 'retro' Memphis/8-bit theme

[EnriqueCanals]

Summary

This PR adds an opt-in theme system to Abbey and ships one reference theme — retro, a Memphis-design / 8-bit / 80s-computer take on the blog chrome. With ABBEY_THEME=default (the default, unchanged behavior) the rendered HTML, asset list, and markdown output are byte-for-byte identical to main. With ABBEY_THEME=retro the whole site picks up the new look without touching any of the default templates or stylesheets.

The intent is to make Abbey usable as a starting point for blogs with a different personality than the minimal default look, without maintainers having to fork the project or maintain a parallel set of templates. If accepted, future themes can plug in by dropping a single CSS file (and optionally view overrides) into the existing folders.

Default theme	Retro theme (ABBEY_THEME=retro bin/dev)

What gets added

Path	Purpose
app/assets/stylesheets/themes/retro.css	Pure CSS, scoped under .theme-retro
app/assets/stylesheets/themes/retro-highlight.css	Rouge palette for retro
app/controllers/concerns/theming.rb	before_action that prepends the theme view path
app/helpers/application_helper.rb	theme_stylesheets helper + :app filter
app/views/themes/retro/	12 ERB variants (layout, nav, footer, admin nav, tags, blog, pages, links, papers)
config/initializers/themes.rb	Theme registry + env var
lib/minimal_markdown_render.rb	Semantic-HTML Redcarpet renderer
test/integration/themes_test.rb	4 tests, 38 assertions
README.md	Themes section

Plus a 1-line include Theming in ApplicationController and a small refactor in Rendering so themes can opt into the minimal markdown renderer.

Zero new gems. Zero changes to Gemfile, package.json, Procfile.dev, config/tailwind.config.js, or any existing default-theme view.

How the theme system works

A theme is a string set via the ABBEY_THEME env var or Rails.application.config.theme (defaults to "default"). When a non-default theme is active a theme can contribute three things:

1. View overrides. app/controllers/concerns/theming.rb prepends app/views/themes/<theme>/ to Rails' view lookup path on every request, so e.g. app/views/themes/retro/blog/index.html.erb overrides app/views/blog/index.html.erb — including layouts and partials. Default theme = no prepend, zero behavior change.

2. Extra stylesheets. Files under app/assets/stylesheets/themes/ are explicitly excluded from the existing stylesheet_link_tag :app bulk-include (via a small override of Propshaft's app_stylesheets_paths in ApplicationHelper) — so they cannot leak into the default build. They are loaded explicitly via a new theme_stylesheets helper, only when the corresponding theme is active.

3. Markdown renderer. Themes listed in Rails.application.config.themes_using_minimal_renderer get a new MinimalMarkdownRender (semantic HTML, no inline Tailwind classes), so they can style content from a wrapper scope (e.g. .prose-retro) instead of fighting the renderer's hard-coded utility classes. The default theme keeps the existing MarkdownRender.

A new theme can be as small as one CSS file:

echo '.dark body { background: midnightblue; }' \
  > app/assets/stylesheets/themes/midnight.css
ABBEY_THEME=midnight bin/dev

Add views under app/views/themes/midnight/ only when you need to change markup. README documents this end-to-end.

What the retro theme contributes

Layer	Highlights
Design tokens	Memphis palette (ink/paper/crt/pink/cyan/yellow/mint/purple/coral), three font families (Space Grotesk for body, VT323 for mono, Press Start 2P for display), retro hard-shadow utilities (shadow-retro, shadow-retro-pink, …), card-retro, btn-retro, tag-retro, crt-window, cursor-blink, marquee, glitch-hover, animate-pop-in — all defined in pure CSS scoped under .theme-retro so they cannot leak.
Chrome	Pixelated SVG favicon, drifting Memphis confetti background, subtle CRT scanlines (under content; opacity tuned for readability), color-block logo, neo-brutalist nav buttons that wiggle on the active page, a BBS sysop admin command line, an arcade-tape footer marquee, and a terminal $ echo copyright bar.
Content	Memphis-style cards with color-cycled hard shadows, rotated date "stickers", glitch-on-hover post titles, and a retro READ MORE button. Body content uses the new minimal renderer + .prose-retro for pixel-display headings, wavy underlines, highlighted <strong>, and terminal-styled <pre> blocks.
Dark mode	Kept as-is via the existing dark class toggle; in the retro theme it reads as a CRT terminal (neon mint on near-black, scanlines stay subtle for legibility).

Compatibility

Concern	Status
Stack	Rails 8.1.x, Tailwind v4, Propshaft — built and tested against current upstream main (da83a8c)
Gem changes	None (Gemfile.lock unchanged)
Default theme	No changes — same HTML, same stylesheets, same renderer
System-test selectors	All preserved (header h1, footer, six nav links incl. new Papers, .fixed.top-0 admin bar, New Post, Sign out, Back to posts) so bin/rails test:system stays green when you flip the env var

Tests

$ bin/rails test                       # 21 runs, 85 assertions, 0 failures, 0 errors
$ bin/rails test test/system/{navigation,blog_public,pages,links}_test.rb
                                       # 15 runs, 32 assertions, 0 failures
$ ABBEY_THEME=retro bin/rails test \
    test/system/{navigation,blog_public,pages,links}_test.rb
                                       # 15 runs, 32 assertions, 0 failures
$ bin/rails test test/integration/themes_test.rb
                                       # 4 runs, 38 assertions, 0 failures
$ bundle exec rubocop $(touched files) # clean
$ bundle exec erb_lint app/views/themes/   # clean

The two pre-existing test/system/feeds_test.rb errors (undefined method 'name' for nil in app/views/feed_posts/_feed_post.html.erb:15) also reproduce on main and are unrelated to this change.

Try it locally

git checkout feature/retro-theme
bundle install
bin/rails db:migrate
ABBEY_THEME=retro bin/dev
# open http://localhost:3000 — toggle dark mode for the CRT variant

Things worth a maintainer eye

• The override of Propshaft::Helper#app_stylesheets_paths lives in ApplicationHelper (not a monkey patch on the gem). Trade-off noted in code comments — if you'd prefer the layout to enumerate stylesheets explicitly instead, happy to take that path.
• The retro theme uses pure CSS rather than extending the Tailwind config — deliberate, so the default build stays unchanged. Theme authors who want to use @apply with custom design tokens can do so by adding a Tailwind entry-point under their theme folder; left out of this PR to keep scope tight.
• lib/minimal_markdown_render.rb autoloads via the existing autoload_lib config — no additional requires in initializers/boot.
• New file count is large but most of it is the retro views/CSS; the actual theme infrastructure is ~250 lines across 4 files.

Happy to iterate on naming (retro vs memphis vs arcade), tone the visuals down, or split this into multiple smaller PRs (theme infra first, retro theme second) if that's easier to review.

[EnriqueCanals]

Pushed 3bffefa to address the "uses pure CSS rather than extending the Tailwind config" trade-off called out above.

Theme design tokens now live in app/assets/tailwind/application.css under a @theme block — Memphis + Grimoire palettes (--color-memphis-*, --color-grim-*), retro hard-shadows (--shadow-retro*), theme-specific font families, and shared animation keyframes. Tailwind v4 reads these and auto-generates the matching utilities (bg-memphis-pink, shadow-retro-lg, font-blackletter, animate-blink, …) along with every variant for free: dark:, hover:, group-hover:, opacity modifiers (text-grim-shadow/40), and arbitrary values (text-[0.62rem], tracking-[0.18em]).

That lets the per-theme CSS files drop the hand-rolled utility duplicates:

• themes/retro.css: 626 → 539 lines
• themes/grimoire.css: 819 → 754 lines
• net: −58 lines, but ~150 lines of duplicated .theme-X .bg-foo-bar + .dark\: / .hover\: / .group-hover\: / arbitrary-value mirrors are replaced by ~90 lines of design tokens

What stays in the theme CSS files is what actually belongs there:

1. theme-root environmental styles (body backdrop, scanlines, scrollbar)
2. a 4-line .theme-X { --font-sans: …; --font-mono: …; --font-display: …; } rebinding so utilities like font-mono resolve to the theme's font stack inside that scope (without touching the default theme)
3. all component classes (.card-retro, .btn-retro, .tome, .spell-btn, .wax-seal, .prose-retro, .prose-grimoire, …), now consuming var(--color-memphis-pink) etc. so design tokens are the single source of truth
4. theme-local keyframes that aren't surfaced as utilities (retro-drift, grimoire-mist, grimoire-sigil-spin, …)

The "default build stays unchanged" guarantee from the original PR is preserved: the @theme tokens only contribute CSS variables on :root (inert) plus utility classes that Tailwind's content-aware extraction only emits when something in the source files references them — and the default theme's views never do.

Verified:

$ bin/rails test                       # 22 runs, 115 assertions, 0 failures
$ bin/rails test test/integration/themes_test.rb
                                       #  5 runs,  68 assertions, 0 failures
$ bin/rails tailwindcss:build          # clean

Net win for future themes: drop new tokens into @theme, rebind --font-*, ship a body backdrop + component classes — no more growing per-theme utility-mimic boilerplate every time a new color or shadow is added.

[EnriqueCanals]

Superseded by #156, which includes everything from this PR plus the drop-in theme refactor (registry, per-theme Tailwind bundles, shared chrome partial, generator, and docs). Closing in favor of the consolidated PR.

[EnriqueCanals]

Closing in favor of #156.