I want my game board to be darker on all squares. Is there a redhotpawn.com setting to change the darkness of the square colors on the game board?

You can change the board settings, yes. Click the "cog" board settings option when viewing the game board. (above submit move button.)

As for the general contrast between light and dark tiles, this isn't as strong as it could be, mostly for historical reasons.*

*When everyone was running low resolution monitors back at the start of the "noughties", we couldn't anti-alias the piece images effectively against all the different board styles, so had to choose lighter-than-ideal dark tiles to mitigate against the jarring rough edges on the pieces. This could be changed now though.

Russ,
Thanks for your help. Under "My Settings," I found "Chess sets, boards and default sizes" allowing me to change the colors of the board by choosing a design color style. I chose the "wood tile" style. I have yet to see the "Cog" choice you referred to.

@zzzzz9200

Sorry, I was referring to the icon's design, there is no cog option itself, just a gear style cog icon representing settings.