message list [nfc]: Move `theme` to BackgroundData.
message list [nfc]: Move `theme` to BackgroundData.

We don't happen to use this in the lower layers that BackgroundData
gets passed down to; but putting it here cleans up the pattern of
what data goes there vs. in top-level props of this component, as
explained in the jsdoc on the type.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile