Use page color for fret number background

[FigmentBoy]

Resolves: #11570

Renders the page background color behind tablature numbers instead of just rendering a white rectangle

• [x] I signed the CLA
• [x] The title of the PR describes the problem it addresses
• [x] Each commit's message describes its purpose and effects, and references the issue it resolves
• [x] If changes are extensive, there is a sequence of easily reviewable commits
• [x] The code in the PR follows the coding rules
• [x] There are no unnecessary changes
• [x] The code compiles and runs on my machine, preferably after each commit individually
• [x] I created a unit test or vtest to verify the changes I made (if applicable)

[FigmentBoy]

I believe the vtest is supposed to fail? Somebody correct me if I'm wrong