How do I change which symbol means what? eg. Open Floor to be displayed as a dot

[danbang]

After changing the font, my Angband (v3.5.1 for Kubuntu Linux 20.04) displays open floor tiles as a square with a dotted outline, the symbol for "this font undefined for this character", rather than the little dot. This is a problem because an empty room will seem filled with squares, rather than being obviously empty. Sometimes I don't notice monsters if their symbol is similar to an empty square, such as a gray capital C or capital O.

I want to change the symbol for "open floor" to another character more suitable for my choice of font. How do I do this?

A web search reveals that this is possible but doesn't give info specific enough that I can do it. There is mention of a "user prefs file" which I am supposed to find in the "lib/customize/" folder, namely "pref.prf", but I can't find these files anywhere. Does that mean I need to create my own "lib/customize/" directory? Do I put this under ~/.angband/Angband ? (as in ~/.angband/Angband/lib/customize ?)

There is no mention of the format of the pref.prf file. If I am to create my own, how do I specify that I want to change Open Floor Tile to 0xB7? (I can't tell what the current character is, but I think it's 0x1F.)

Thanks for any help you can give.

Dan Bang