First question how much do you know about CSS? CSS is fairly simple but you can break a lot of stuff if you are not careful... What you will have to do is edit the CSS to match your site. Your best bet in my opinion atleast for the actual forums them selves it to do something similliar to this
background-color: transparent; /* you can make this what ever you want it to be */
border: 1px #0000; /* pick your actual color here */
color: #FFFF /* pick actual color here */
border: 1px #0000;
PHPBB has over 2,000 lines of CSS through out various .css pages. you can download each one to your local machine and edit each one as you wish, or go to the ACP-> Styles-> Themes -> Your Theme-> Edit and then edit the css there as you see fit.
Something that might help you is, if you open up the inspect element in your browser and navigate to the specific part of the forum you want to edit you can look for the actual css code so for example
your nav bar is located in
looking for the css you find that the css code for it is
/* all the CSS code here */
If you want to match the background image find this line of code
/* all the code for your style */
/* or */
If you have any questions let me know.