CSS Loader for Millennium

Make CSS Loader work with Millennium's randomized CEF debugging runtime—without
-dev mode or a separate fixed CDP endpoint.

Problem

CSS Loader itself—not its themes—is incompatible with Millennium's CDP runtime. Both projects use Steam's CEF debugging interface, but they use it differently. Millennium starts its debugging interface on a randomized port that stock CSS Loader cannot discover and follow. Stock CSS Loader instead expects its own stable, externally reachable Chrome DevTools Protocol (CDP) endpoint and creates Steam's .cef-enable-remote-debugging marker to request that path. Running Millennium normally does not give stock CSS Loader the stable CDP connection it expects.

Solution

CSS Loader for Millennium solves this by avoiding the randomized-port connection entirely. The modified backend resolves themes and publishes their runtime state to the companion plugin, which runs inside Millennium and applies the styles through Millennium’s controlled interfaces—preserving CSS Loader’s native themes, profiles, and configuration model.

Note

This is not merely a fix for delayed theme loading. Without a compatibility layer, Millennium's randomized CDP runtime and stock CSS Loader's stable-port connection model cannot interoperate, so CSS Loader does not function correctly in the first place.

Main highlights

• Full CSS Loader configuration model: themes, profiles, dependencies, patch controls, colors, generated variables, enable state, and cascade order.
• Direct CSS delivery that preserves inline SVG, data URLs, animations, local assets, and advanced CSS exactly as authored.
• Desktop, Big Picture, Quick Access, Main Menu, notification, and popup target routing through Millennium’s supported in-process interfaces.
• Normal Millennium operation with no Steam -dev mode, external debugging port, or .cef-enable-remote-debugging marker.
• Overlay behavior that keeps Fluenty, SpaceTheme, Pebble, or another selected Millennium theme active underneath CSS Loader.
• Desktop management for installed themes, profiles, settings, store browsing, downloads, updates, and the always-on Windows backend.
• Reliable downloaded-profile support, including Unicode and emoji configuration values, bundled image-picker assets, and dependency themes.
• Accurate Store installation reporting that surfaces dependency and download failures instead of displaying false success messages.
• Hardened Windows packaging with complete native imaging support and clean, reproducible backend builds.
• Single-instance desktop manager that restores and focuses the existing window when launched again.
• Atomic runtime updates: the companion accepts new state only when its revision and content hash match.
• Compatibility and reliability coverage for real-world themes, Unicode profiles, profile assets, unsafe paths, nested inline-SVG filters, and /themes_custom images and fonts.

How this resolves the conflict

CSS Loader for Millennium replaces the incompatible injection path while retaining CSS Loader's theme engine and configuration behavior. The backend still reads existing CSS Loader themes, profiles, dependencies, patch values, colors, components, class translations, and activation order.

Stock CSS Loader path	Millennium-compatible replacement
Creates .cef-enable-remote-debugging	Does not create or depend on the marker
Requires an externally reachable CDP port	Publishes resolved injects to the Millennium companion
Cannot follow Millennium's randomized debugging port	Runs inside Millennium and uses its controlled plugin interfaces
Injects every Steam document through external CDP	Uses an in-process Millennium overlay plus its controlled per-plugin CDP proxy for isolated BrowserViews

The companion layers CSS Loader over whichever Millennium theme is selected. Desktop and Big Picture are synchronized directly inside Steam; Quick Access, Main Menu, and notifications use Millennium's isolated per-plugin API because they live in separate BrowserViews. The runtime does not expose an external CDP port, recreate the .cef marker, require Millennium -dev mode, or run a separate browser bridge.

Eliminating the delayed theming flash is an additional benefit of this design, not the entire purpose of the project. The compatibility runtime persists the latest resolved injection state on disk, and its companion starts as part of Millennium instead of waiting for an external standalone CDP connection.

Additional Highlights

• Replaces CSS Loader's external CDP injection path with ordered, direct style injection inside Millennium.
• Live visual change updates without needing a steam reload/restart.
• Uses existing themes from ~/homebrew/themes; no manual conversion required.
• Preserves profiles, dependencies, patch options, colors, CSS variables, local images/fonts, class translations, and CSS cascade order.
• Defaults to overlay mode, keeping Fluenty, SpaceTheme, Pebble, or another selected Millennium theme active beneath CSS Loader.
• Preserves each resolved inject as CSS text instead of flattening it into a rewritten bundle.
• Bundles the backend and the separately maintained CSS Loader Companion for Millennium inside one desktop installer.
• Bootstraps a new %USERPROFILE%\homebrew\themes library automatically; Decky or a pre-existing CSS Loader installation is not required.
• Republishes direct runtime state when settings or watched CSS files change.
• Keeps Desktop, Big Picture, Quick Access, Main Menu, and notification targets separate, matching CSS Loader's real document routing.

Compatibility baseline

The first full audit used Decky CSS Loader as the source of truth:

Themes	Controls	Tested states	Target snapshots	Mismatches
42	120	595	2,380	0

That matrix covered every selectable option plus component and color states, with CSS parsed by Steam's Chromium engine. See the full methodology and known reference warning in Compatibility verification.

Installation

Important

This project currently targets Windows and requires a working Millennium installation. The stock CSS Loader standalone backend is not the correct runtime for this setup; this Millennium-aware backend and its companion must be installed.

1. Install Millennium and CSS Loader Companion 1.2.0 or newer from its separate release.
2. Download and install the latest MSI from this repository's Releases page.
3. Open CSS Loader for Millennium. First-run setup creates the theme library, registers the bundled onedir backend for login startup, and publishes a valid empty state.
4. Leave your preferred Millennium theme selected; CSS Loader is layered over it.
5. Restart Steam once, then manage themes, profiles, and every patch option from the desktop app.

The MSI installs the onedir backend beside the desktop app and registers its launcher in the current user's Windows login autorun key. It does not bundle, install, or update the separately released companion. The desktop app creates %USERPROFILE%\homebrew\themes when it does not exist. The backend publishes the resolved state and the companion applies it inside Steam.

The backend stores its generated runtime mailbox inside the installed companion at Steam\millennium\plugins\css-loader-companion\runtime. It does not create or select a Millennium theme. See Clean installation for the complete first-run contract and migration behavior.

Build from source

Requirements

• Windows 10 or 11
• Python 3.11+
• Node.js 20+
• Rust stable and the Microsoft C++ Build Tools (for the MSI)
• Millennium and Steam for end-to-end testing

git clone https://github.com/DevsNate/CSSLoader-Millennium.git
cd CSSLoader-Millennium

python -m venv .venv
.\.venv\Scripts\python -m pip install -r runtime/backend/requirements-dev.txt
npm ci --prefix apps/desktop

npm run verify
npm run build:release

The MSI is written beneath apps/desktop/src-tauri/target/release/bundle/msi/. Individual commands are available for build:backend and sync:desktop.

Repository map

Path	Purpose
runtime/backend	CSS Loader compatibility logic and direct-state publisher
apps/desktop	Tauri theme manager and bundled installer
tools/audit	Reference capture, parity, and Steam class-map auditing
docs	Architecture and verification methodology
fixtures/parity	Policy and location for ignored local audit output

For the full runtime flow, see Architecture.

Status and scope

This is an early Millennium-focused distribution. The verified baseline is strong, but Steam UI updates can change minified classes and BrowserView behavior. Reports should include the Steam channel, target view, theme/profile, and exact option that differs. See Contributing.

Themes are third-party content and are not bundled here. Each theme remains under its author's license. This repository is derived from the CSS Loader runtime and desktop projects; see Attribution and provenance.

Project repositories

Repository	Purpose
CSSLoader-Millennium	Desktop app, CSS Loader-compatible backend, installer, runtime publisher, and verification
CSSLoader-Companion-Millennium	Millennium Marketplace plugin that applies resolved CSS Loader injects inside Steam

License

Distributed under the GNU General Public License v3.0.