These are the Theme Songs that I think fits the character. Please be noted that these are all Vocaloid Songs.

LET'S ALL LIST IT! :D

●Axis
Germany - Cantarella
Italy - Hello, How are You
Japan - 1925
Maria Sakura Honda - Senbonzakura
Kuro - Six Trillion Years and Overnight Story

●Allies
America - Super Hero
England - GLIDE
France - Madness of Duke Venomania
Russia - Matryoshka
China - Panda Hero
Canada - Tokyo Teddy Bear

●Central Europe
Spain - Dancing Samurai
Romano - Brother is Worried (WHAT THE FUCK!? XDDD)
Austria - Cantarella ~grace edition~
Hungary - Kocchi Muite, Baby
Prussia - Law-evading Rock
Belgium - Gift of the Princess who brought Sleep
Netherlands - Cendrillon
Ukraine - Karakuri Pierrot
Belarus - The same as Russia's theme.
Romania - Sadistic Vampire
Liechtenstein - Wolf who fell in live with Red Riding Hood
Switzerland - Same as Liechtenstein.

●Nordics
Sweden - Same as Monaco.
Denmark - Childish War
Norway - Childish War
Iceland - Crystalline

●Baltics + 1
Lithuania - Witch
Latvia - Kokoro
Estonia - Eye Examination
Poland - I Can Take Off My Panties!

●Asia
Hong Kong - Yí Èr Fanclub
Vietnam - Donut Hole
Taiwan - Butterfly on Your Right Shoulder
South Korea - Invisible
North Korea (For fun~) - Judgement of Corruption

●The Rest
Monaco - Poker Face
Australia - WAVE
Egypt - Paradachichlorobenzene
Turkey - 8HIT
Greece - Ah, it's a Wonderful Cat Life
Seychelles - Deep Sea Girl
Sealand - Triple Baka
Wy - Eh? Ah, Sou

●OCs
Hiroshima - Rolling Girl
Nagasaki - Lost One's Weeping
Moscow - iNsANiTy
Siberia - Zeema (This is not a Vocaloid song, and anyone who guessed this meaning and whose song is it, I'll literally follow you immediately.)
Atlantis - Deep Sea Girl
Shuzuya(Kyoto) - The Tailor Shop in Enbizaka
Berlin - The Muzzle of Nemesis

If I missed a OC or Hetalia character, please tell me immediately.