COSMIC Theme Switcher

COSMIC panel applet that toggles between light and dark theme with a single click on a systray icon.

It works by flipping the content of $XDG_CONFIG_HOME/cosmic/com.system76.CosmicTheme.Mode/v1/is_dark (default: $HOME/.config/...) between true and false, which is what COSMIC watches to switch modes.

Build dependencies (Pop!_OS / Debian)

sudo apt install cargo cmake just libexpat1-dev libfontconfig-dev \
    libfreetype-dev libxkbcommon-dev pkgconf

Install

From the repo root:

just build          # cargo build --release
just install        # copy binary + .desktop + icon into $HOME/.local
just reload-panel   # restart cosmic-panel so it rescans applets

Then open COSMIC Settings → Desktop → Panel → Add applet and add "COSMIC Theme Switch".

To uninstall:

just uninstall

Make sure $HOME/.local/bin is on your PATH; the .desktop file relies on that for its Exec=cosmic-theme-switcher entry.

To install somewhere else (e.g. system-wide), override PREFIX:

sudo just PREFIX=/usr/local install

Run without installing (smoke test)

just run    # cargo run --release

On the bar

Layout

• src/theme_io.rs — read/write the is_dark file (unit-tested)
• src/icons.rs — embedded Sun/Moon SVGs
• src/app.rs — cosmic::Application implementation
• src/main.rs — entry point
• data/ — panel metadata and icon assets