@3gi8ma welcome to the forum.
No. The Markdown editor was changed between v3.0.x and v3.1.x and some css classes were also changed. This is not a bug as custom css is an unsupported feature. Explained a bit better here.
You can either rewrite your userchrome.css file for the new editor or revert to what is now the legacy editor.
Tools> Options > General > Advanced > Use the legacy Markdown editor
