Under Settings → Appearance pick Light, Dark, High contrast or Match system, or make your own theme. Themes change Tenonkit’s colours, not your websites.

Theme files
A theme is a .tenontheme file you can share. Double-click one to install it. Any colour it leaves out comes from the light or dark base:
Ocean.tenontheme
{
"format": "tenonkit-theme",
"version": 1,
"name": "Ocean",
"author": "You",
"description": "Deep blue",
"appearance": "dark",
"colors": { "ground": "#0b1a2a", "surface": "#10243a", "accent": "#4fc1d6" }
}