edit: updated after the discussion in this merge request
- allow a different coloring of switches, check/radios, progressbars, spinners, suggested action buttons and headerbar .selection-mode if needed by adding specific color vars for these widgets
- the color of those widgets is NOT CHANGED and they still use $selected_bg_color and $fg_color (spinner)
- allow a different sizing of the headerbar and buttons, switches, entries inside the headerbar if needed by adding specific metrics
- the size of these widgets is NOT CHANGED and they sill look exactly the same like in current 3-24