Mstdn.ca does have other colour schemes, but they work slightly differently as it runs a special version of Mastodon called Glitch Mastodon.
To change the theme on Glitch servers like yours:
1. Click ⚙️ Preferences
2. Click Flavours (at left of screen)
3. Choose Vanilla Mastodon or Glitch Edition depending on whether you want Glitch's extra features
4. Choose "skin" at bottom of screen
5. Click "use this" below that