How do you remove this grey bar that’s on the back of the tabs themselves?
You start to post a screenshot, but somehow failed. Please edit your question to provide it. Also, add your OS name and LO version.
Are you really experiencing the issue in Calc (I doubt it because you can’t enter tabs in cells), Impress, Draw (for same reason), Base (tabs are not used) and Math? Please retag, removing everything except tabs and writer.