Hey!!
I bought the Klippe T 4 Ultramarine as well, and I was under the same dilemma.
I got two keycap sets to go with it :
1) Ducky Joker - For gaming (It's great for the 50$ price and I won't mind ruining them by mashing the keys in a year or two)
2) GMK Dolch 2 - The groupbuy is live right now, and it's 100$ (cheap for a GMK set). Black/Grey/White go well with any base color imo. You could also opt for the "mist" version of Dolch 2, as it has blue legends.
I'm new to this hobby so I don't comment much, but I'm quite proud of the theme I'm going for with my PC setup so rest assured that I put a lot of thought into this
PS:This is gonna be my first build and I already spent 450+ euros on it XD
Cheers!