styling: Fix h5 colors in dark mode

After adding dark mode in (#164), the fixed-color override for `fg-color--light`
no longer works as expected since the color can now be one of two options.

For now, delete this override and live with what's provided by the default styling.
This works better than the current broken styling in dark mode where h5 headers
show up as super dark text on a dark background.

Signed-off-by: jaydee-coder <187227976+jaydee-coder@users.noreply.github.com>

Author: jaydee-coder

docs/stylesheets/extra.css

@@ -5,7 +5,6 @@
   --md-accent-fg-color:          #D3506F;
   --md-footer-bg-color:          #2C2C2C;
   --md-footer-bg-color--dark:    #2C2C2C;
-  --md-default-fg-color--light:  #000000B0;
 }
 
 /* Heading overrides as the default ones are very small */