docs: increase contrast in dark-theme

The contrast-ratio between the foreground and background colors aren't
quite ideal in dark mode. This improves the situation a bit, by making
the body-text a bit brighter.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9043
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23236>

docs/mesa3d_theme/static/screen.css_t

@@ -96,3 +96,7 @@ div.citation > p:last-of-type:after {
 }
 
 {%- endif %}
+
+[data-bs-theme="dark"] {
+  --bs-body-color: #dee2e6 /* $gray-300 */;
+}