Theme dropdown: should be fixed, so dropdown doesn't look bad (wide breakpoint)

[kelseyscherer]

When choosing a theme with a short name, dropdown fits to length of short name. Instead, is should have a fixed width to avoid dropdown looking like this:

[kelseyscherer]

Still seeing this issue, on all dropdowns now.

When dropdowns are selected:

When nothing is selected:

[kelseyscherer]

Dropdown also resizes to theme name on blueprint pages, and ideally it would be fixed everywhere it appears.