Code editor gutter does not respect dark theme colours

In dark mode, the code editor's gutter/breakpoint margin could use an incorrect background colour instead of the active editor theme colours. This made the strip beside the code lines look out of place and reduced readability in dark-themed editors.

Expected: the gutter uses the active dark theme resources and repaints when the editor theme/highlighting is applied.

Actual: the gutter could keep a stale/default brush and fail to match the dark editor theme.