This is more of a styling issue, I'm actually not 100% sure how to fix it, though I think people with the same style have brought it up in the past.
The styling there is 100% standard XF styling so the problem exists with the style, more than anything.
IIRC, the editor background is transparent by default so you will want to set the editor background to be a colour which matches your normal background colour.
There are style properties for this... I think Style Properties > Editor > Editor Background