Hovering on text editor themes/font size should show preview

[brollb]

When setting the theme or font size, hovering on the options should show the user the editor with the given setting applied.

For example, in the following image, the theme should be set to "Twilight" while it is hovered upon in the context menu. Clicking out of the menu w/o selecting it should revert the theme back to the currently selected theme.