1 · Make a folder

A theme is a folder with a manifest.json: values only, no CSS and no scripts. Start from ArcadeCore's own look. In a terminal on the arcade's own machine:

arcade-cli new theme my-theme

That makes ~/my-theme with every value named and a README. A name is 2 to 32 lower-case letters, digits and dashes.

2 · Pick the colours

Open ~/my-theme/manifest.json and change the colours under look.light and look.dark. Each has the same eleven:

"look": {
  "light": { "window": "#f5eadc", "panel": "#fff8f0", "text": "#1d1d1d", "accent": "#b85c2e", ... },
  "dark":  { "window": "#2b2622", "panel": "#211d1a", "text": "#ece4da", "accent": "#d0773f", ... },
  "font": "system", "radius": 3 }

Leave dark out to use the light colours in both. Words must stand out from what they sit on: checktheme (step 5) refuses a pair too faint to read an address in, and says which one.

3 · A font (optional)

look.font is one of system, serif, rounded, mono. Or put a font of your own in the folder as files/name.woff2 and name it there. look.radius rounds corners, 0 to 12 pixels.

A theme holds at most 16 files and 1000 kB, so other arcades can fetch it quickly.

4 · Try it on

See it in this machine's browser only, as the folder is now. Change the folder and reload to see it again.

trytheme ~/my-theme

Nobody else sees it. trytheme off goes back.

5 · Check and install

checktheme ~/my-theme
installtheme ~/my-theme

checktheme says what is wrong, or the theme's id: a hash of every file, so a changed theme is a new id. installtheme keeps it under that id; it shows nowhere yet.

6 · Wear it

settheme my-theme        this arcade's look, for everyone who has not picked one
allowtheme my-theme      people may pick it in Settings → Theme…

settheme none goes back to ArcadeCore's own look.

7 · Share it

Offer it to the arcades this one links with: Share a theme here, or

sharetheme my-theme

Each of their operators sees it under Shared by others and chooses whether to install it; nothing installs by itself. It travels over the mesh, checked against its id on arrival, and since a theme is colours and a font only, no code goes with it. sharetheme my-theme off stops offering it; arcades that installed it keep it.