Returns the CSS color variable for the given color index.
Uses Theia's --theia-scmGraph-* variables, mirroring the VS Code
scm graph color scheme:
0 (current ref) → historyItemRefColor
1 (remote ref) → historyItemRemoteRefColor
2 (base ref) → historyItemBaseRefColor
3–7 (default rotation) → foreground1–5
Returns the CSS color variable for the given color index. Uses Theia's
--theia-scmGraph-*variables, mirroring the VS Code scm graph color scheme: 0 (current ref) → historyItemRefColor 1 (remote ref) → historyItemRemoteRefColor 2 (base ref) → historyItemBaseRefColor 3–7 (default rotation) → foreground1–5