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Themes
Themes set your group's colors. They're at Manage → Themes.
Light and dark
You set both, side by side. Members see whichever their device is using, so it's worth checking your group in both — a color that reads well on white often doesn't on black.
What each color does
The list covers the whole page: backgrounds and text, your primary and secondary colors and their accents, positive and negative colors for confirmations and warnings, headers, muted text, panels, and the player.
Pick your primary and secondary first and look at the result before going further. Most groups only need those two and their text colors; the rest are there when you want them.
Colors can be transparent, which is what borders and overlays use.
Following the site
Reset to Site Defaults clears your colors, and your group follows the site from then on — including any later changes to the site's palette. That's different from setting your colors to today's defaults by hand, which keeps them exactly as they are now.
Contrast
The site's own palette is built so text stays readable against its background. Once you change a color that's yours to keep up — if you darken a background, check the text on it is still easy to read.