The most prominent theme of Money Heist is 'Resistance'. It is the main reason behind their heists. It is engraved in their masks and in their costumes also.
Spoilers ahead, if you don't want to get the details before you have completed the series, I just warned you!
...Like when they are having a drink together in flashback...
When they were having a a drink, they were singing "Bella Ciao", which they sing a few more times or the tune is played in the background. Bella Ciao is an Italian protest song, sung by the workers in protest against their higher authorities. Later it became the theme song during "....Italian resistance movement against the Nazi German forces occupying Italy, and, during the Italian Civil War, the Italian partisan struggle against the fascist Italian Social Republic and its Nazi German allies.Versions of "Bella ciao" continue to be sung worldwide as an anti-fascist hymn of freedom and resistance."
Later, it was revealed that, Professor's grandfather was a part of the Italian Resistance and he had taught the Professor the song, and later the Professor teaches it to the team. That's why he says: We are The Resistance(meaning protesting is in their blood).
Now, they are protesting against what?
...Tokyo also narrates that the professor's life revolved around only one idea - Resistance...
Professor planned the heist in the first place to avenge his father's death, he got the idea of robbing banks from his father. He was sick and was getting treated in a hospital when he was a child. His father tried to rob a bank in order to give the Professor the best treatment, but was killed in a shootout.
The Professor explains their(robber's) action to Inspector Raquel Murillo later:
In 2011, the European Central Bank made €171bn out of nowhere. Just
like we’re doing. Only bigger. Do you know where all that money went?
To the banks. Directly from the factory to the pockets of the rich.
Did anyone call the European Central Bank a thief? No. ‘Liquidity
injections,’ they called it. I’m making a liquidity injection, but not
for the banks. I’m making it here, in the real economy.
They are protesting against the existing system of economy. If the Government prints currency,the rich gets richer while the poor belongs to where they belonged earlier and nobody calls the Government criminal. They are also printing money, which didn't exist earlier. So they are not robbing anything. This is the way they are resisting the Government by doing the exact same thing. The Professor had meticulously planned everything for this kind of 'resistance' which Tokyo was referring to.