Quicker theme browsing

Right now, switching between themes is a matter of 2 clicks, which add up fast when browsing between different themes and seeing what users like.

Using the Up/Down arrow keys while the dropdown is focused causes the dropdown to open, but doesn’t switch to the next/prev theme in the list for quick viewing.

Suggested approaches

1. Hovering a theme applies it immediately for preview. Currently the dropdown is using a native OS select list, which may not expose hover events on each item, so I’m not certain this would work.
2. Using Up/Down arrow keys with the dropdown selected applies the next/prev theme in the list
3. Add up/down arrow buttons next to the dropdown to apply the next/prev theme immediately