If you don't like the default look of LibreSprite, DON'T PANIC, there's plenty of community made themes that make the app looks awesome.
Well, most of the available themes can be found here. These are all made by and for the community.
Neutral Dark - by Blackraven
Purple Tint - by Maujin111
Unofficial Dead Cells - by Maujin111
Simple, download the .zip archive, extract and copy to the /data/skins part of the directory where libresprite.exe is.
Then open LibreSprite up, go to Edit, Preferences and go to the Theme tab.
Select the theme that you just put into the /data/skins folder, and click Select in the bottom left.
Restart LibreSprite to see the changes.
Well, just go back to the Themes tab in Preferences, and select a different theme and do the same process as before.
NOTE: DO NOT DELETE THE FILES BEFORE YOU CHANGE THE THEME If this does happen, simply copy the original, call it the same as the deleted theme and launch LibreSprite again.
If this doesn't fix it, then, you'll need to reinstall the theme and try again the steps above, or reinstall LibreSprite.