Add a CSS rule to catch accelerators in menuselection and guilabel roles

[parulin]

Problem

Sphinx allows us to tell the user about the accelerator key in :menuselection: and :guilabel: roles. A new HTML <span> element is created but the CSS stylesheet doesn't apply any style to it, so the information is lost when looking at the web page.

Reproducible Project

Use any of this role:

:menuselection:`&File --> &New`
:guilabel:`&File`

Build the theme and check the output, the letters F and N don't have any style.

Expected Results

Having a line under the letters will provide useful information to the user.

Environment Info

• Python Version: 3.11.2
• Sphinx Version: 8.2.3
• RTD Theme Version: 3.0.2