HighContrast: Honour .monospace set by GtkTextView

Copy the line that sets a monospace family from Adwaita (same place too)

noticed at #141 (comment 91610)
1 job for gtk-3-22 in 9 minutes and 22 seconds (queued for 28 seconds)
Status Job ID Name Coverage
  Build
passed #19984
build

00:09:22