On the websitehttps://streak-stats.demolab.com/demo/ you can open Advanced Options to get a UI for changing the color query parameters. You can add properties and then select a color for each one you add.

After filling out your username and other options, you can use your custom colors by copying the url/markdown.

If you want to contribute a theme, at the end, you can click Export to PHP which will display the PHP code needed to be added tothemes.php. More info here -#32