Duolingo Themes Settings - This is an open source extension that will allow you to change the color scheme on the Duolingo.com.
Duolingo Themes Setting - This is an open source extension that will allow you to change the color scheme on the Duolingo.com website to ready-made ones. And you can also customize the color scheme for yourself by turning on manual settings.
Github repository - github.com/skilloff1337/Google-Extension-Duolingo-Themes-Settings