So I'm pretty new to the lost grove, I only unlocked it about a month or two ago, but is there certain place where it is easier to find themes? I know they're pretty rare in general but I have been trying for weeks to find some and haven't had any luck. I have a few perfumes but would prefer not to have to use them all up.

Perfumes increase the chances of finding a theme, but they are costly and I prefer to sell them for money, but that's up to you.

Second, if you release a bunch of pets over 42 days of age, it increases your chances of finding an egg, but they aren't guaranteed to be themed.
I personally buy and release pets specifically for that purpose, although a lot of people are against it.

Overall, finding a theme in LG is pretty rare and I've only found around 3 in 2 years of playing. (Then again, I'm not that active in LG)

Hope this helps! :)