Changes:
● Support translucent base colors. Also, inactive.alt.base.color is supported now.
● Optimization in searching for stylable toolbars.
● Added the key spread_header for making simple headers (and used it in some themes).
● Merged the background of a header with that of its parent view for a more natural look.
● Fixes for RTL, especially in header views.
● Fixed a potential cause of crash in combo-boxes with hard-coded styles.