Rollup merge of #97089 - GuillaumeGomez:improve-settings-theme-display, r=jsha

Rollup merge of #97089 - GuillaumeGomez:improve-settings-theme-display, r=jsha

Improve settings theme display

This is a follow-up of #96958. In this PR, I changed how the theme radio buttons are displayed and improved their look as well.

It now looks like this:

![Screenshot from 2022-05-17 20-46-20](https://user-images.githubusercontent.com/3050060/168887703-a01e3bd5-9644-4012-ac11-2ae7bacd6be6.png)
![Screenshot from 2022-05-17 20-46-12](https://user-images.githubusercontent.com/3050060/168887707-132f8b2d-1163-462f-b7dd-f861121bdee7.png)

You can test it [here](https://rustdoc.crud.net/imperio/improve-settings-theme-display/doc/foo/index.html).

r? `@jsha`