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
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.
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