Using another website's design?

You need someone to check the providence of the design, if it is a template (often the case with Wordpress) then so long as your avoid any directly infringing elements (like logos) then you may be able to modify the theme or template. However, I suggest you add your own refinements, such as a legible font) stylistic elements to reinforce the brand (eg colors logos), and trademark any unique elements. Note that many websites use libraries for the RESTful APIs or PHP page generation and if these are open-source, then the basic coding is not protected (except to extent license requires attribution) but other elements like database compilation or user-contributed comments may have stronger or weaker IP provisions.