Smart Contracts created with NiftyKit (as of April 3, 2022) use the EIP-2981 Royalty Standard.
EIP-2981 is a standardized way to retrieve royalty payment information for non-fungible tokens (NFTs) to enable universal support for royalty payments across all NFT marketplaces and ecosystem participants.
Opensea currently doesn't support the EIP-2981 standard. Royalty Percentages need to be set manually on Opensea. Navigate to the "Collection Editor" on Opensea and adjust the Percentage fee field under Royalties, as shown below.