Try going into the knowledge menu (~), e for feature knowledge, select one of the walls, hit 'v' for visuals and find a solid block. Then you can either set them all in the visuals editor and save it to a pref file, or just note the number of the solid block, and then go into lib/pref/font-win.prf and change lines like
by replacing the 127 with the solid block number.
Code:
F:56:1:127 F:57:1:127 F:58:1:127 F:59:1:127
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