The names of Miku Append's voicebanks are a little metaphorical, so here's the literal description of each one.
Light: strong, high
Vivid: strong, middle
Solid: strong, low
Sweet: soft, high
Soft: soft, middle
Dark: soft, low
So if you're inspired by Dark, you want to do a soft low tone.
For soft vocals: more whispers/breathy, more quiet. You may need to get closer to the microphone. After recording, experiment with different resamplers, since there can be a dramatic difference in how they handle breathy sounds. I personally prefer tn_fnds but you might like other resamplers more.
For low vocals: aka deep, thick, "masculine". The opposite of a cutesy high voice. Practice using your chest voice when singing and pronouncing vowels with a more open mouth.