Let's assume that my client (C) has a corporate website, created by an agency (A). A created a pretty simple, yet well designed website which uses plain HTML and no backend to edit any of the data. C has to issue (and of course pay) A every time they want any changes to their website which has become pretty expensive. Now C asks Me to write a dynamic module that integrates into the existing website. This module should be an event-planner that enables C to change at least that part of the website on his own.
I am a little bit worried whether it is ok for me to simply take some of A's graphics and HTML/CSS to use on my project (since I need the new module to be a part of the website). Is there any legal stuff to be considered? Who's property is the already existing website? Can one make a general assumption at all or is this something that necessarily needs to be issued in a contract?