Highlighting text is not an “action” on text. It is an additional attribute on a character.

To achieve your goal, you must first define a character style with the specific colour selected as Background. Give a name to this style. You can now give it a keyboard shortcut.

This shortcut is not auto-toggling like Ctrl+B or I. I suggest you also associate a keyboard shortcut to character style Default Style so that you can cancel the effect of highlighting from the keyboard.

To show the community your question has been answered, click the ✓ next to the correct answer, and “upvote” by clicking on the ^ arrow of any helpful answers. These are the mechanisms for communicating the quality of the Q&A on this site. Thanks!