If anybody wants to use a different theme in Sonarr v5 apart from the built-in light and dark themes, there are several themes at Sonarr 🐳🔥⚠️ - theme.park Docs . I used a browser extension called Stylus to install a theme but there are other installation methods. For the Stylus installation method, install Stylus first. Then if you want to use the aquamarine theme for example, write a new style and put this code into it:
@import "https://develop.theme-park.dev/css/base/sonarr/sonarr-v5-base.css";
@import "https://develop.theme-park.dev/css/base/sonarr/aquamarine.css";
See discussion at Sonarr v5 themes · themepark-dev · Discussion #742 · GitHub