I'm looking for a certain piece for building my set in LDD. I've already looked through the extend theme, but I can't find it. If anyone knows where to find it, it would be super cool.
I assume you are talking about Lego Digital Designer since you mention "extend theme". Also assuming the design ID is correct (6163991) according to your screenshot and also the brickset database, searching for it in LDD in the search bar with updated bricks database and indeed in extended mode does not produce any results. I think it safe to conclude that the element is not present in the LDD bricks database. This happens, perhaps this will be corrected in a following database update but those only happen very sporadically anymore. Could be that the design ID is incorrect, which could be figured out by using the "Replace parts" service of lego, but this service isn't available this time of the year.
This piece is available in the 4.3.11th version of LDD in 10th set category