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