Refactor Valo main menu styles into separate mixins

Add a new utility mixin for defining width-ranges and height-ranges.

Refactored ValoThemeUI to use Responsive and styled it to fit in a
variety of viewport widths.

Fixed an issue with “flat-dark” using named colors, which trips the
“type-of” function.

Theme “theme select”.

Change-Id: Iff55ca2c23d2e279b6199590db2fdfe51423c1df