Yeah, I know it sounds kinda dumb but ever since I got to sylestia, I never understood what tagged themes are.

the "Tag" refers to the bit in a pet profile's genetics section that says what the theme is, for example, this pet has a gold star and purple "Sunbather Gazania Nytekrie", or the blue star+text in this pet's profile.
These denote them as being Themed.

However, in common vernacular on the site, "Tagged theme" typically is used to specifically refer to the gen1 pets like the first one we linked, with the gold star/purple text. This is because once upon a time, Purebreed pets like the second one we linked were not tracked by the site- they did not have any official recognition as being themed, and could not be searched for via Advanced Search.
Hence the distinction between the so-called Tagged Themes and the Purebred Themes was more clear, and it's still used, although often to the confusion of players who joined in more recent years when both have the theme tags.

Ok, thanks