Save one of the available theme files as theme.ron in the appropriate directory for your operating system:
$HOME/.config/gitui/theme.ron(MacOS)$XDG_CONFIG_HOME/gitui/theme.ron(Linux using XDG)$HOME/.config/gitui/theme.ron(Linux)%APPDATA%\gitui\theme.ron(Windows)
Alternatively, you can save both the light and dark theme files in the same directory and switch between them by passing the theme name with the -t flag:
gitui -t cyberdream-light.ron