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