fix: Change sidebar backgrounds to dark gray

[DeMoorJasper]

  Open in CodeSandbox Web Editor | VS Code

Changes the black sidebar to be dark gray, matching the actual codesandbox editor

Before:

After:

CSB Editor for reference:

[kettanaito]

I've done this color change on purpose because if the sidebar and the editor are the same color, it's really hard to understand what file tab is currently selected. I've shared this feedback with our designers as well, and, imo, this is something we should change in CSB rather than in this theme.