why is it 'zelený' and not 'zeleného' (the accusative case)?
zeleného is the genitive form for masculine and neuter nouns, and the accusative form for masculine animate nouns.
For masculine inanimate nouns like čaj, the accusative form stays identical to the nominative: zelený
Why isn't "Oni chtí zelený čaj" been accepted?
Because "chtí" doesn't exist :D
This is how "chtít" conjugates in standard Czech:
There are more forms in dialects (especially for the 3rd person plural), but I don't believe "chtí" exists in any dialect :D
I get this because I copied it before. I only remember 'tea'.