[Documentation:Forum] Update Forum Dark Mode Documentation (#344)
[Documentation:Forum] Update Forum Dark Mode Documentation (#344)

* Update forum.md

* Update forum.md

* Update forum.md

Co-authored-by: Barb Cutler <b4fc851c82a06ed3e5f5c72e00b8d2b8b8ad995c@users.noreply.github.com>
Submitty/submitty.github.io,Submitty/submitty.github.io,Submitty/submitty.github.io,Submitty/submitty.github.io