Based on a thread in Wish List, in Scrivener for Windows 3.0, there is a bug with how accelerator keys are displayed in themes other than the default light theme. In Windows, there is a convention that pressing the Alt key will cause menu entries to underline the accelerator keys for each option (if they have them). Pressing Alt again will remove the underlines. This works as expected in Scrivener 3.0 with the default theme, but it does not do so with most of the other themes I have tried. In those, the underlines for the accelerator keys are always present.
There appear to be a number of other oddities in the non-default themes as well – I noticed that in the Grey Matter (Light) theme there was no selection bar in the menus.