Open LibreSprite.

Go to Edit/Preferences/Theme, you can see there are 2 paths at the top of the Available Themes list.

For example on windows:

-- C:\Users<your-username>\AppData\Roaming\Libresprite\data\skins\ --

-- C:\Program Files\Libresprite\data\skins\ --

or linux:

-- /home//.config/libresprite/data/skins/ --

-- /tmp/.mount_LibreShxiPnk/usr/bin/data/skins/ -- (You should need be root user to modify this directory)

Choose 1 of them then click Open Folder. Copy the file you've just downloaded here and extract it.

Open the Preferences/Theme dialog again, choose a theme from the list, click Select then click OK. Restart Libresprite to make the change happen.