Lizzie now supports custom theme, but its requirements may be a bit high and require a jar file.
I think it may be easier for more users to use image files or configuration files directly.

I made a simple solution that would allow you to customize the Theme directly from external images. Of course, this is only the first step. You can only modify the background, the board and the stone, and will support more UI settings in the future.

Here's an example, here I'm trying to define a look like Sabaki, of course just similar. I really like the appearance of Sabaki, so I used Sabaki's image file directly. If it is an offense to the original author, I will delete it.

The configuration method is as follows:
a) Change the 'theme' value to 'Custom' (which is a fixed value) in the config.txt file:
"theme": "Custom",
b) Add a new item "custom-theme" to specify your theme name:
"custom-theme": "",
for example:
"custom-theme": "sabaki",
c) Create subfolders in the lizzie folder, attached please find the sample theme (theme.zip, I really like the appearance of Sabaki, so I used Sabaki's image file directly. If it is an offense to the original author, I will delete it.):
/theme/custom/
for example:
/theme/custom/sabaki
d) The custom-theme folder must contain the following images, the names must be the same, feel free to use your favorite images:
background.jpg
board.png
black0.png
white0.png
e) Run the Lizzie. Enjoy it.

The modified Lizzie file and more sample files, I published a pre-release for testing. Welcome to test and suggest: (This Pre-release may be updated frequently)
https://github.com/zsalch/lizzie/releases/tag/v0.5%2B