"Mám tmavě červenou hrušku."

Translation: I have a dark red pear.

[przemek1ni]

I've never seen a red pear before

[Janmunroe]

So both tmave and svetle are "adverbs" and therefore don't need to be in agreement with the word they're modifying (i.e. "tmavou/svetlou cervenou hrusku".)? Now I have to reach back into my dim past and fetch up what exactly and "adverb" is. Hmmm... No rest for the weary.

[Chel184386]

Is it common among native speakers to barely pronounce the first letter/phoneme of a word? The voice here barely pronounces the 't', and I'm finding I have to listen very closely to hear many of the prepositions as they tend to be half pronounced and run into the next word.

[VladaFu]

It is common to connect prepositions with the subsequent word. In all languages I know.

You should not skip any letters except some specific cases (jsem -> sem, jsme -> sme). You may find it difficult to hear some consonants, but they have to be there and a native speaker would immediately find it missing or would not understand you.

[GinaHearn4]

Another example of using tmavě, an adverb instead of an adjective. Can someone explain why, please?

[VladaFu]

It is an adverb and modifies the adjective modrý. English often works the same way.