fix(ui): set dark colour for FAQ text (#2962)

[previous](#2934) reverted the
custom text colour, but because it didn't specify a replacement colour,
the text reverted to the default light mode colour. This change sets the
default text colour for dark mode.

Author: hogo6002

docs/_sass/color_schemes/custom_dark.scss

@@ -7,6 +7,8 @@ $new-body-background-color: #292929;
 // Overriding dark default for just-the-docs styles
 $color-scheme: dark;
 $body-background-color: $new-body-background-color; // Replacing default $grey-dk-300
+$body-heading-color: $grey-lt-000;
+$body-text-color: $grey-lt-300;
 $link-color: #C9F5F7;
 $nav-child-link-color: $white; // Replacing default $grey-dk-000
 $sidebar-color: $new-body-background-color; //Replacing default $grey-dk-300