various UI fixes

[undefined-box]

Hey there,

Found this project in the GitHub suggestions and it's very nice. I forked it and applied some quick fixes, while I was at it I looked at some issues and there was a nice feature request too. I leave a pull request here, in case it could be useful.

Keep up with the good work!

feat: add hint on main toolbar (issue #60/#62)
feat: highlight selection on color settings to improve UX
fix: vertical aligns toolbar on desktop devices
fix: avoid color palette overflow on "Color Settings" menu
fix: center icons on main toolbar