You will need to change almost everything to make it similar to Facebook. Note that what I am going to explain to you now, may need some of your own basic CSS knowlegde.
Before you start, you should make a backup of the original theme!!
We will mostly need the base.css file to change things like background, header, fonts etc. This is pretty easy. I have a quick trick too see which thing you need to edit to get the effect. Look at the next picture.
http://www.mijnbestand.nl/Bestand-YTAP4UIPBYB4&download=WBYF7ISPKS74
You will first have to go to your site. Than do right-mouse-click on the thing you want to change. Click on "element inspecting" (in chrome, don't know if this works in any other browser!). Now you will mostlikely see the image of the screenshot that I have made. When you go with your mouse over the lines in the earea with the bleu arrow, you will see some things in your site will light up.
I have taken the Header as an example. When you are on the link: "<div class="ow_header">" the header will light up. Click on that. On the right hand there you can see the CSS file. (base.css for now). The yellow arrow says on which line and which css file are the preferonce of the header. (you can check that by yourself, by going to the theme.).
Now try to untick some of the boxes who are marked with the red arrow. Look at what happens!
You are now disabling / temporarily deleting the lines. When you untick "background image" you will see that the header is turned to white. What is happenning is, that the background image is deleted and the background color is set enable.
If you change the color of the background color (you need to do this manual by using notepad++ and opening the base.css, in your theme folder) to the same blue as the facebook has, than we have finished that one (these are hex codes, search for them on google). (don't forget to delete the line of the backgroundimage! otherwise it won't have any effect!!
You can play a lot with this trick!
You can do this too for the register "table". Just right-click on the table and redo the steps I told you.
Note: you will have to add some lines, without any CSS knowledge this could be very hard. The trick I told you is not the best, but is the easiest way of creating a theme.
You will have to something like "align=right;" or "position_fixed=right". But sometimes this won't work, becuase they could have been edited in the PHP file. I haven't looked for that now.
Last but not least, there is the Graphics folder in your theme. Check that out. For changing some the graphics you can easily create a new icon / image and replace it with the original one (make sure that it has the same name, extension and size!) That will always work.
Last note: It really doesn't matter in how much you will "copy" the style of Facebook. Take that from me. You won't get as much users as Facebook gets, if you only change the style. You will need to have something special that others don't have, and what makes people to visit your site, to get it popular!
That only applies to you, if you want it to be popular, ofcourse.
I wish you all the best with the theming! If you have any more questions than please let me know!
Great_Day