theme: Stylesheet papercuts and other visual fixes

• sync general label styles from libadwaita
• further clean up of existing modules' stylesheets
  • ibus candidate
  • nm-dialog
  • screenshot
  • popovers
  • tiled-preview (removed, merged with misc.scss)
• remove redundant spacing variable as its equal to padding
• improve drawing functions further
  • update button drawing functions coloring and :checked state styles
  • update entry function, use with shell search entry
• add osd button style definitions, use with osd elements
• make the overall bg_color less dark

Fixes related to #4969 (closed)