Dark mode for doco
Dark mode for doco

Documentation should be legible when users/developers have dark mode on.

Refs #19737
laagesen/moose,laagesen/moose,sapitts/moose,harterj/moose,milljm/moose,jessecarterMOOSE/moose,lindsayad/moose,lindsayad/moose,jessecarterMOOSE/moose,milljm/moose,dschwen/moose,bwspenc/moose,bwspenc/moose,harterj/moose,andrsd/moose,andrsd/moose,dschwen/moose,idaholab/moose,harterj/moose,sapitts/moose,laagesen/moose,idaholab/moose,lindsayad/moose,lindsayad/moose,milljm/moose,harterj/moose,sapitts/moose,milljm/moose,sapitts/moose,jessecarterMOOSE/moose,dschwen/moose,andrsd/moose,bwspenc/moose,sapitts/moose,jessecarterMOOSE/moose,milljm/moose,lindsayad/moose,andrsd/moose,bwspenc/moose,dschwen/moose,idaholab/moose,idaholab/moose,bwspenc/moose,andrsd/moose,dschwen/moose,harterj/moose,idaholab/moose,laagesen/moose,laagesen/moose,jessecarterMOOSE/moose