Fix RichTextInput toolbar appearance

[fzaninotto]

The RichTextInput has several issues:

• the disabled version doesn't have the same buttons (and therefore the same default size) as the enabled version
• the button icons are not legible enough
• the small variant of the toolbar is too large for a desktop UI
• the sx prop isn't supported

This PR fixes them all.

• Fix UI glitches
• Update screenshots in documentation

Before

After