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
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.