generate swatches for visualizing theme colors

[zoof]

I have a fork (https://github.com/zoot/maptoposter) where I added seaborn to pyproject.toml and have a script (theme-swatches.py) that creates a png file that illustrates the colors in each theme (sorry, I'm not familiar with making pull requests). I found it useful to be able to see the colors in existing themes in order to create my own themes.

[rcsmit]

typo in URL :)

(https://github.com/zoof/maptoposter)