@nathan-mitchell,
Thanks for reaching us. What you are trying is correct and I just tried it locally by changing the Attribute values in the same page, it worked here. Are you up to date with the Darkest Dark theme? (it gotta be CI4 version).
Also, if you go to the “Darkest Dark” preferences page under General > Appearance > Darkest Dark, you can switch between different editor color themes. Does that make any difference?
If not, it’s possible that something is persisting your preferences (I would say oomph plugins). So removing the oomph preferences in your <home dir>/.eclipse folder could make it work.
Let us know if this helps. Cheers!
Aaron Lara