Include the Lepton theme as an option

[trickypr]

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Discussed in https://github.com/pulse-browser/browser/discussions/112

^{Originally posted by black7375 September 26, 2022} Hello, I'am author of Lepton custom theme. I accidentally found your work and looks fun.

What do you think of containing my theme in your browser? It is the most popular custom theme used by quite a few people, and is also used in various forks such as Warterfox(G4, G5) and Floorp browser, RunningCheese.

[trickypr]

This is going to be postponed to the next release because of the following:

• [ ] We need an api for theme_experiments to provide userContent.css equivalent
• [ ] A way for themes to define custom about:config prefs