Theming: Enable unified color for splitters, toolbar borders and toolbar separators

The current themes use distinguished colors for movable splitters, toolbar borders and toolbar separators. Dark themes do not draw any border between toolbar and pane content.

The UI face lift defines the same color for the above splitters, borders and separators.

This requires requires special handling in the styling code, since up till now, the border colors were calculated based on the theme colors (in order to blend better with an optionally recolored toolbar, see -color feature). For the new use-case, all colors need to come straight from the theme.