[BUGFIX] remove superfluous slash from path to grideditor.css

Change-Id: I22da2b0519fd3e8793bb3ecb2f49d13f16fdd4a9
Resolves: #58788
[BUGFIX] Background image must be set to important to override original

Change-Id: I492b237c2a6406a89fbbaf8dda8ed205ffe1339f
Resolves: #58641