The closest I could find to what you’re asking for is the LibreOffice Extensions Repository. I did find some things relating to line margins and fonts, but they may not be exactly what you’re asking for.

Another site I found was LibreOffice Templates. This isn’t affiliated with LibreOffice or The Document Foundation, so exercise caution when downloading templates. Again, I’m not sure if this is exactly what you want, but I tried my best to help.