Hi Krzysztof,
Boards go here as .svg images: picochess/web/picoweb/static/css/chessground/board.
Piece sets go in here: picochess/web/picoweb/static/css/chessground/pieces. Create a new folder for every piece set with twelve .svg files in it (bB bK bN bP bQ bR wB wK wN wP wQ wR).
Then you need to include the names of the new piece sets in these files (if you search for existing pieces like 'merida' or 'leipzig' you will find the right places):
configuration.py
picochess.ini.example-web-aarch64
picochess.ini.example-web-x86_64
picochess.ini.example-dgtpi-clock
web/picoweb/static/css/custom.css
web/picoweb/static/css/chessground/theme.css
web/picoweb/static/css/base.css
web/picoweb/static/js/app.js
web/picoweb/templates/settings.html
web/picoweb/templates/clock.html
You must also add the names of the new boards to the boardThemes array in web/picoweb/static/js/app.js and create new .css files for them in picochess/web/picoweb/static/css/chessground.
Greetings,
DJ