Probably the adverbalized noun bisschen, which is derived from bite. As it is adverbalized, there is no capital.
About bisschen Duden online says meaning etwas or ein wenig it is mostly found with ein, functioning as an adverb [my translation]. Duden lists bisschen as a pronoun, not all of which take a capital letter.
Extra confusion here, if you're looking for a noun to apply ein to, is that it could be seen as applying itself to kalt. However, es ist kalt casts kalt as an adjective.
My advice is, just learn that ein bisschen can be used this way.
What about "It is somewhat cold today"? It was not accepted.
I know that ein bisschen means "a little", but cannot "somewhat" mean the same as "a little" in this context? (I guess that would be written as Es ist etwas kalt heute, but I still think the meaning would not be significantly different...)
You are right. It would be "Es ist etwas kalt heute." and the meaning would not be changed changed significantly.
At least that's how it feels to me, as a German native. ;-)
I don't know if Duo is a bit too picky here or if it just hasn't thought about this possible translation.