Theme Toggle applet for COSMIC™

A one-button panel applet for the COSMIC desktop that switches between dark and light mode and shows you which one is active.

Light mode	Dark mode

• ◐ — light mode is active; click to switch to dark
• ◑ — dark mode is active; click to switch to light

It watches COSMIC's theme-mode config, so the glyph also flips when the mode is changed from Settings → Desktop → Appearance or by the sunrise/sunset auto-switch, not only when you click it.

Right-click opens a small menu: a dark/light switch, every theme file found on disk (with a check on the one currently applied), and a reset-to-COSMIC-default entry. Picking a theme applies it the same way Settings → Import does — the mode switches to match the theme's palette, and your transparency/frosted-glass settings are kept.

Theme files

Theme files are the .ron exports COSMIC itself uses — what Settings → Appearance exports, what cosmic-themes.org serves, and what Tweaks for COSMIC saves. Drop them in

~/.local/share/themes/cosmic/

(/usr/share/themes/cosmic/ is also searched, as is the Tweaks flatpak's sandboxed copy of that directory). The file name is the theme name.

Emacs themes

contrib/cosmic-theme-from-emacs.el converts any installed Emacs theme into a theme file, so the desktop can match the editor:

emacs --batch -l contrib/cosmic-theme-from-emacs.el -f cosmic-theme-from-emacs-batch \
      doom-tomorrow-day doom-tomorrow-night modus-vivendi

It reads the theme's face colours (default, mode-line, link/cursor, success/warning/error; doom themes' own bg-alt/highlight where present), decides dark or light from the background, and writes ~/.local/share/themes/cosmic/<theme>.ron on top of COSMIC's stock palette — the same recipe as the themes on cosmic-themes.org. Themes are located through your package directories (elpa/, straight/build/) without loading your init file; pass -L DIR for themes elsewhere. Pair it with something like auto-dark in Emacs and both directions stay in sync.

Command line

The same binary doubles as a CLI, handy for keyboard shortcuts:

cosmic-ext-applet-theme-toggle --list            # themes found, with dark/light and path
cosmic-ext-applet-theme-toggle --apply "Dracula"  # apply one by name

Install

Requires a Rust toolchain and the usual libcosmic build dependencies (libxkbcommon-dev, libwayland-dev, pkg-config on Debian/Ubuntu/Pop!_OS).

git clone https://github.com/ransomts/cosmic-ext-applet-theme-toggle
cd cosmic-ext-applet-theme-toggle
cargo build --release

# per-user
install -Dm0755 target/release/cosmic-ext-applet-theme-toggle ~/.local/bin/cosmic-ext-applet-theme-toggle
install -Dm0644 res/io.github.ransomts.CosmicAppletThemeToggle.desktop ~/.local/share/applications/io.github.ransomts.CosmicAppletThemeToggle.desktop

# or system-wide
sudo install -Dm0755 target/release/cosmic-ext-applet-theme-toggle /usr/bin/cosmic-ext-applet-theme-toggle
sudo install -Dm0644 res/io.github.ransomts.CosmicAppletThemeToggle.desktop /usr/share/applications/io.github.ransomts.CosmicAppletThemeToggle.desktop

If you have just: just build-release && sudo just install (or just prefix=~/.local install for a per-user install).

Then add Theme Toggle to your panel in Settings → Desktop → Panel → Configure panel applets.

How it works

COSMIC keeps the mode in a single config file, ~/.config/cosmic/com.system76.CosmicTheme.Mode/v1/is_dark (true or false). The applet writes that file through cosmic-config and subscribes to changes on it, the same way COSMIC's own settings daemon and apps do.

Applying a theme writes the ThemeBuilder to com.system76.CosmicTheme.{Dark,Light}.Builder and the built Theme to com.system76.CosmicTheme.{Dark,Light} — the same two configs Settings and Tweaks write. Nothing else is stored; the theme list is rescanned each time the menu opens.

Menu actions close the menu first and apply ~300 ms later. Changing the theme while an applet popup is still mapped makes cosmic-panel stop compositing its applets until it is restarted (observed on COSMIC 1.8 / Pop!_OS 24.04); the delay sidesteps that.

If auto switch (sunrise/sunset) is enabled in Settings, a click is a temporary override until the next transition — same as toggling in Settings.

License

GPL-3.0-only. The panel-button layout mirrors cosmic-panel-button from pop-os/cosmic-applets.