Designing images for brand which already have identity

Hello,

I am not myself a designer, but i enjoy putting things together, but my real question is, How do you design for a brand which already have identity? This brand website is already have very nice graphics from someone else but i need to create extra web pages and i wanted to also add graphics to it. But somehow i cannot get the same feeling from my designs. What kind of details you are looking for the having same style of designing and keeping the design style same. Thank you so much for helping! ❤️