Previously, the radio button choices for themes would wrap awkwardly on
narrow screens. With this change, the group of choices will prefer
bumping down to the next line together, leaving the setting name on its
own line.
Also fix some minor spacing issues:
- Align the setting name vertically with the radio button choices.
- Use margin instead of padding for most spacing choices.
- Use no margin/padding on the right-hand side.
This reduces the number of clicks required to change theme.
Also, simplify the UI a bit (remove setting grouping), and add a "Back"
link close to the settings icon.