i would start by first using the element inspector that comes by default in firefox and chrome.
First - turn off all caching in your boonex site.
Second - go to profile page and right click anywhere on a block - just not on an image or link or video. There should be an option called "inspect" - click that/
This will open a section that has all kind of weird looking things. you are concerned with the section that has css attributes and the page html coding. In the css section, you can make changes to the page and see what it looks like without making any changes to code.
Then, you are just going to have to play around with the css to get what you want. Once you have what you want, look to the right of where you edited the css stuff and you will see a file name - something like: common.css:113. common/css is the file name you are going to edit. Now, the one you will need to look in first will be in the templates/tmpl_whateveryourcustomtemplatenameis/css/general.css/ The number after the : is the line number. If you do not find that line number or it is another bit of code - then you can add your changes to the bottom of that css file.
I hope that makes sense, but here is where you will most likely have an issue. The template for the page blocks is used all over the site - so if you make changes to one page block, it will also affect many other page blocks that use the same template/ so, as an example - there is a pageblock called disignBoxFirst. This is used for almost all of the blocks - including the profile blocks. So, if you make the profile blocks transparent - most likely almost all other blocks will become transparent as well.
So, you or the template creator would need to create a new pageblock template with a transparent background. then figure out how to assign those blocks to the profile page.
It can be done - it may just take some time.