This has been around since a few snapshots. Scoreboards are impossible to see in the sidebar.

How to reproduce issue:
Start any world or pre-existing.
Either create an objective or use an existing one and then setdisplay as that objective.
Add yourself a point to that score.

You will find it hard to see as it lacks the proper opacity and needs a solid background color. Even playing around with the text options for chat or accessibility options doesn't help much (while you can barely make out what the score text is, the score itself is even harder to see)

Recommend not only fixing, but perhaps allowing scoreboard sidebar text to have its own settings?