Linux: tray icon is a fixed-color icon and does not follow the panel light/dark theme

[fnune]

Description

On Linux (KDE Plasma 6 Wayland) the tray icon stays dark on a dark panel, so it is hard to see. The package ships only full-color apps/Cap.png icons with no monochrome or -symbolic variant, so the tray host has nothing to recolor. Would a symbolic/monochrome tray icon be possible, so the panel can theme it?

Reproduction

1. KDE Plasma 6 with a dark panel theme.
2. Run Cap so the tray icon appears.
3. The icon stays dark regardless of the panel theme.

Additional Context

• Cap version: 0.5.2
• OS: CachyOS (Arch), KDE Plasma 6 / KWin Wayland
• libayatana-appindicator 0.5.94

[linear]

CAP-740

[tembo]

The tray icon code lives in apps/desktop/src-tauri/src/tray.rs, with icons stored in apps/desktop/src-tauri/icons/tray-*.png.

This is closely related to the macOS fix in PR #1579, which converted the tray icons to monochrome PNGs and used the macOS template icon API (set_icon_as_template(true)) to let the system adapt colors automatically. That approach works for macOS because the template API inverts monochrome icons based on menu bar appearance.

For Linux with libayatana-appindicator, the theming mechanism is different. Desktop environments like KDE Plasma and GNOME expect symbolic icons — typically SVGs with a -symbolic.svg suffix installed to standard icon theme paths (e.g., /usr/share/icons/hicolor/scalable/status/). The panel then recolors these based on the current theme. Tauri's tray-icon crate on Linux doesn't have an equivalent to icon_as_template; instead, applications either:

1. Ship symbolic SVG icons and set them by name (not by embedded bytes), letting the system locate and theme them
2. Provide both light and dark icon variants and switch based on detected theme
3. Use a high-contrast monochrome icon that works on both backgrounds

Option 1 aligns with how most native Linux apps handle this. It would require adding SVG symbolic icons to the build/packaging and using TrayIconBuilder::with_icon_name() instead of embedding PNG bytes on Linux.

Related: #1553 is tracking Linux desktop support more broadly.