Settings: General

Two tabs: Settings and Appearance. The window opens on Settings.

Settings

System tray icon: Cosmic Capture Kit will remain in the system tray for easily launching capture sessions.; A small helper process that stays running so a capture is always one press away. On macOS it is a menu bar item, on Windows and Linux a tray icon.; On by default on macOS and Windows, because the global capture keys only work while it is running. Off by default on Linux, where your desktop owns the capture key instead, so nothing breaks without it.
Automatically start on login: Brings the tray helper back after a restart.; On by default. This row only appears while the tray icon setting above is on, since there would be nothing to start otherwise.

System Default: Disable to customize the theme.; On by default. While it is on, the app follows your desktop's own theme and the four rows below are hidden.
Light/dark mode: Automatic follows the system's light or dark preference.; Automatic, Dark or Light. Automatic by default.
Accent: A row of colour swatches. The first one hands the choice back to your system accent colour. After the nine theme colours there is a + button that opens a Custom Accent panel where you can type an exact colour as hex or RGB, drag a hue slider, or pick one you used recently. That panel has Apply, Use theme default and Cancel.; By default no accent is set, so the app follows your system.
Automatic Contrast Boost: Adapts your selected accent color for optimal contrast.; On by default. Keeps a chosen accent readable against the background rather than letting it wash out.
Edge rounding: Three preview boxes: Round, Slightly Round and Square.; Round by default on macOS and Linux, Slightly Round on Windows, so the app matches the shape of the desktop around it.
Selection box thickness: How thick the line around your capture region is drawn, from 1 to 8 pixels.; 2 by default. This one is always available, even with System Default on.

Three sliders, each from 0 to 100 percent. They control how much the app dims the parts of the screen you are not capturing. Higher means darker.

Setting	Default	When it applies
During Region Selection	66%	While you are drawing and adjusting a region
During Countdown & Recording	33%	Once the timer starts and while recording, so you can still see what is happening
During Preview	90%	Behind the editor when it opens as an overlay