dont need knowledge of php
you do need to know at least the hex color, or find a rgb-hex convertor, then you can change colors...
to change the layout you need to know HTML, its all the .tpl files in /template/subSilver (or other theme)
probably your best bet it to find a theme with a layout you like, and edit the colors/images to what you like. make sure you keep any of the theme authors copyright but add something like "edited by
www.dfhq.net"
(usually this is in phpbb_footer.tpl)