gtk3: Add public color for text views

Based on upstream commit:
https://gitlab.gnome.org/GNOME/gtk/-/commit/c09b9904c87c232bba78cf96553afad3cde21c29

Fixes #80

Author: jnsh

common/gtk-3.0/3.24/sass/_colors-public.scss

@@ -31,6 +31,7 @@
 @define-color link_color #{"" + $link_color};
 
 @define-color content_view_bg #{"" + $base_color};
+@define-color text_view_bg #{"" + $base_color};
 
 //WM
 $wm_highlight: lighten(opacify($header_bg, 1), 3%);