fix #8541 (Inline editor content renders on top of gutter when scrolled horizontally)

[JeffryBooher]

Fixes #8541

This pr merges into kevin's inline-styling branch so this should be merged before #8529

[JeffryBooher]

@dangoor

[dangoor]

Hmm... This undoes a change that @larz0 made in #8362

[dangoor]

It's not clear to me why @larz0 changed the background for .CodeMirror-linenumbers to be transparent in brackets.less, but this change does fix #8541 and looks fine to me in both light and dark themes.

[larz0]

@dangoor it's transparent before you added the code below which does a good job at overriding line number bg in brackets.less so it's Jeff's change is totally fine.

.dark {
…
       .CodeMirror-scroll {
            .CodeMirror-linenumbers {
                background-color: @dark-bc-inline-widget;
            }
        }  
}

[dangoor]

@larz0 I added the transparent because it was in your dark UI PR, but if you're cool with it and everything checks out, that's great!

This is already merged into the dangoor/inline-styling branch so it's ready to go.