> thanks for the answers
>
> I don't know what Metapost or CSS box-shadow are (yet...)

MetaPost is the programming language used to describe every symbol and
line that Therion draws. In your "layout"-"endlayout" section of your
configuration, you add a "code metapost" and "endcode" section, then you
add the code needed to redefine the cave walls that you are using. It's
complicated, since it is a programming language. Takes some practice.

https://therion.speleo.sk/downloads/thbook.pdf - look for the "New map
symbols" section, which shows some examples.

Loads more examples:
https://therion.speleo.sk/wiki/metapost

And if you want to see all the MetaPost code that Therion uses by
default, see:
https://therion.speleo.sk/wiki/metapost#how_to_get_therions_metapost_code_and_tex_code
This is where Martin was suggesting you look for the line that looks
like this:
def l_wall_pit_AUT (expr P) =

I'm still in the process of learning the language myself, and am not yet
good enough to write what you need.
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