Talking about general truths but within a context that uses past tense

Solution 1:

"unaware" has no tense. It adopts the past tense of "strolled." See what happens in the present continuous tense:

Pikachu is strolling along the street blissfully unaware that the Earth is round.

When the verbal phrase is in the past tense, the reported thought becomes past.

Pikachu strolled along the street blissfully unaware that the Earth was round.

Tenses in indirect speech are explained here [BBC]

Solution 2:

I prefer "was round".

Reasoning

Pikachu was under a misapprehension. He thought the Earth was flat.

We can't say, "Pikachu thought the Earth is flat" because it isn't. Therefore we aren't describing a general truth. We have to say, "Pikachu thought the Earth was flat."

In order to match tenses we say, "Pikachu was unaware that the Earth was round."