The problem is that, in using the Styles menu to underling, you’re bypassing Tinderbox’s rather complex logic for managing light and dark appearances. The following is the more idiomatic way to do what you want.
The menus/shortcuts for bold/italic/underline/etc. are a toggle. Use to turn on, use again to turn off. If working in a selection where the style is question is only partly present you might need to use the command twice.