wordpress site inside a frame

hi,
yes, this is my first post, since i gave up after looking for solution (for something i thought of being very simple problem) for more than 2 days without success.
here i uploaded a sketch to show my problem, and here is brief text description;

basically i want to create a site using wordpress but inside a given frame while trying to keep it simple with the coding and whatnot.
i would be grateful if someone can share his or here experience on this.
should custom theme be made or would it be more simple to try and use something like iframe and force 2 sites to show as one?
or should i just give up on this and go with some regular solution and maybe change a background? ??
i am making this for free (for a little project of my friends and me). up until now i have only made few basic sites using wordpress so my knowledge is very modest.