Themes
With inbuilt themes, you can customize the look of the card without doing any manual customization.
Use &theme=THEME_NAME
parameter like so :

All inbuilt themes
Codeforces readme stats comes with several built-in themes (e.g. dark
, radical
, merko
, gruvbox
, tokyonight
, onedark
, cobalt
, synthwave
, highcontrast
, dracula
).
You can look at all available themes from the theme config file & you can also contribute new themes if you like :D