gtk3: Use darker text_view_bg for dark variant

Earlier commit [1] added the text_view_bg public color, but simply
applied the base background color to it. Use the $dark_sidebar_bg
in dark variant to achieve better contrast, as that is the intended
purpose for the public color.

[1] 2c726e4

Author: jnsh

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

@@ -31,7 +31,8 @@
 @define-color link_color #{"" + $link_color};
 
 @define-color content_view_bg #{"" + $base_color};
-@define-color text_view_bg #{"" + $base_color};
+@define-color text_view_bg #{"" + if($variant != 'dark', $base_color, opacify($dark_sidebar_bg, 1))};
+
 
 //WM
 $wm_highlight: lighten(opacify($header_bg, 1), 3%);