I'm doing this for myself, but I would be happy to make a list for anyone that needs it!
This is a collection of lists I've made of themes with particular...themes. For instance, for a time, I wanted to collect themed pets that have a Forest theme, so I looked for all themes with forest-, tree-, and leaf-related names. But I can make a list of any kind, for instance Chocolate themed themes, Firefly themes, Moon themes, all blue Bulbori themes (all blue themes of all species, however, is simply too broad.) Lists of relatively small event themes, like a list of all Easter themes, would be possible, too.

If you've ever wanted to collect themes that are related to your username, I can provide a personalized list!

Even if I don't get requests, I might just make these lists for fun. I like making lists.
These lists are just names; pictures would be possible, but would take quite a while.
Although I certainly try to be thorough, I might miss a theme or two in a list. I don't mind being notified one I've missed.

My first list, as an example (note: I interpreted some themes as forest and others not. It's not a straightforward category):

Forest and Tree themes:
Winter Evergreen Aeridini

Falling Leaves Aurleon
Forest Sunlight Aurleon
Frosted Forest Aurleon

Forest Shadow Bulbori

Autumn Forest Draeyl
Evergreen Draeyl
Forest Wanderer Draeyl
Goblin Grove Draeyl
Withered Leaves Draeyl

Blue Spruce Faelora
Evergreen Faelora
Forest Fungi Faelora
Haunted Woods Faelora
Holly Tree Faelora

Forest Caller Ferrikki
Frosted Forest Ferrikki
Red Cedar Ferrikki
Woodland Sprite Ferrikki

Redwood Griffi
Winter Pine Griffi

Flourishing Mangrove Kelpari
Gilded Forest Kelpari
Ice Forest Kelpari
Turning Leaves Kelpari

Winter Pine Lighira

Frosted Pinecone Luffox
Jungle Vine Luffox
Leaf Chaser Luffox
Maple Luffox
Woodland Luffox

Forest Spring Lunemara
Tree Dryad Lunemara
Wandering Willow Lunemara
Wilted Willow Lunemara
Woodland Lunemara

Fallen Tree Lupora
Forest Floor Lupora
Forest Guardian Lupora
Frosted Pine Lupora
Ruby Wood Lupora
Tree Moss Lupora
Wood Satyr Lupora

Forest Guardian Morkko
Forest Wisp Morkko
Grove Warden Morkko
Sleeping Forest Morkko
Spooky Woods Morkko
Woodland Rider Morkko

Autumn Leaves Nephini
Forest Afternoon Nephini
Twilight Grove Nephini

Autumn Foliage Ny'vene

Frozen Forest Nytekrie
Leaf Pile Nytekrie

Evergreen Puffadore

Ethereal Leaves Qitari
Fallen Leaves Qitari

Fallen Leaf Ryori
Frosted Leaf Ryori
Woodland Wanderer Ryori

Mysterious Forest Sylvorpa
Summer Forest Sylvorpa

Ghostly Woods Vulnyx

Forest Fairy Zolnixi
Royal Maple Zolnixi
Wooden Twilight Zolnixi

Some theme list ideas: Star themes, Ice and Snow themes, Sunset themes, Dawn themes, Weather themes, Ocean themes, Wind themes, Mushroom and Fungi themes, Swamp and Bog themes, Fire themes, Elemental themes, Night themes, Fruit themes, Gemstone themes, Night themes

Themes based on real-life bird species:

Emperor Penguin Aeridini
Spotted Woodpecker Aeridini

Blue Grosbeak Aurleon
Christmas Robin Aurleon (European Robin?)
Flamingo Aurleon
Holly Cardinal Aurleon (Northern Cardinal)
Hummingbird Aurleon (Ruby-throated Hummingbird)
Kingfisher Aurleon
Penguin Aurleon (Unknown penguin)
Racket-tailed Roller Aurleon

Tufted Titmouse Ferrikki

No Griffi? Except maybe:
Snow Hawk Griffi

Turtle Dove Lunemara

Sunlit Bluebird Lupora (Unknown Bluebird species)

Robin Morkko (Unknown Robin species)

Horned Owl Nephini

Barn Owl Nytekrie
Kingfisher Nytekrie
Hummingbird Nytekrie
Peacock Pheasant Nytekrie
Red Hawk Nytekrie
Red-winged Blackbird Nytekrie
Snowy Owl Nytekrie
Wild Turkey Nytekrie

Paradise Parrot Puffadore (extinct species)

Kestrel Vulnyx

Cloud, Rain, and Storm themes:

Rain Cloud Aeridini

Rainy Day Aurleon
Spring Storm Aurleon
Tempest Winds Aurleon

Stormy Sea Draeyl

Stormy Moss Ferrikki
Sunset Clouds Ferrikki

Arctic Storm Griffi
Freezing Rain Griffi
Red Skies Griffi
Thunderstorm Griffi

Storm Guardian Lighira

Autumn Rain Luffox

Thunder Moon Lunemara

Spring Tempest Lupora

April Showers Morkko
Autumn Storm Morkko
Tidal Storm Morkko

Catatumbo Lightning Nephini
Winter Storm Nephini

Lighting Struck Ny'vene

Storm Herald Nytekrie

Hurricane Qitari

Steel Skies Vulnyx

Autumn Storm Zolnixi
Spring Showers Zolnixi

WIP
Chocolate themes:

Chocolate Candy Bulbori
Mint Chocolate Chip Bulbori
White Chocolate Bulbori (technically counts?)

Chocolate Egg Draeyl

Moon themes:

Autumn Moon Aeridini
Spring Moon Aeridini

Moonlight Chill Bulbori

Moon Rising Draeyl

Moon's Kiss Faelora

Moonlight Luffox

Bottled Moonlight Lunemara
Moon's Light Lunemara
Moonlight Magnolia Lunemara
Moonlit Sea Lunemara

Blood Moon Lupora
Full Moon Lupora
Howling Moon Lupora
Moon Stalker Lupora

Equinox Moon Morkko

Blood Moon Nephini
Moonlight Magic Nephini

Haunting Moon Ny'vene

Harvest Moon Nytekrie

Frozen Moon Puffadore

Hallowed Moon Sylvorpa

Moonlight Magic Zolnixi

---------------------------------------

Star themes:

Star-crossed Griffi

Summer Stars Kelpari

Celestial Lights Lighira
Heavenly Nebula Lighira
Starry Night Lighira

Snowflake Stardust Luffox?

Star Gazer Nephini

Stargazer Qitari

Star Breather Ryori

April Lyrids Vulnyx
Shooting Stars Vulnyx

Starscape Zolnixi