Fix navigation theme option alignment

What does this MR do?

Fixes the alignment of the navigation theme options in User Settings > Preferences > Navigation Theme when the name of the theme is long (i.e. "Dark mode (alpha)").

The options now expand to use the entire width of the container, and labels that are too long are wrapped.

Screenshots

	Before	After
Screenshot
GIF

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Related to &2902 and #14531 (closed)