how to change logo colors to match site's theme?

There's no definite answer. It depends on the software you're using. I see you're using SMF, so my first guess would be to find a file titled smflogo.x or logo.x where x is .png, .jpg, .gif ...

If you have a linux shell you can use 'find . | grep -e gif -e png -e jpg | grep logo'... download that image and edit it in gimp.
Also you can do,
find . | grep -e png -e gif -e jpg

If that doesn't work then turn to google.

EDIT: i had a quick skim of your source,
/Themes/cargo_20rc4/images/custom/logo.gif

Open the file /Themes/cargo_20rc4/css/index.css and change the #logo a block to your new image.

lol ur lazy mofo

lol trying to learn everything , so far barely scratching the surface on everything as well lol. im pretty good with smf now so next would be themes and how they work edits etc. next would be mostly hosting concerns and seo stuff. im redoing everythign one last time arafgalk;fdhs

this would fit

this theme is 10X worse than the other one.. you need your site to have a professional feel to it, you dont use a dark theme.. dark themes r for like clan websites, 15yr olds mypsace profiles, artistic stuff.. not what your doing.. deff switch the theme back to the lighter one, and drop the arcade.

anyways the way a theme works for a CMS is they have a file called a css file, that includes things like where the pictures go, what color fonts and links are, that kinda stuff.. its alot of hard core coding garbage that you probably dont need to learn, for the most part you can get by with just switching the logo around.

once you get your theme changed around, you really need to start thinking about advertising and how your going to attract enough people that want to use this site, for it to ever make you any money.. you need to think about how it could possibly ever generate money, and if it will ever get there.. things like selling add space and stuff will only break even on a great idea after like 3 years.. most anything your gonna wanna try and do to make some money off thsi site is gonna take a HUGE view base, like 10 thousand a day.. and without a MASSIVE ad campaign, thats never gonna happen..

i dont wanna be a downer and burst your bubble, but your idea has already been done by craigslist and ebay.. with that and how late your getting into the .com game, you have virtually no chance of ever making anymore than like 5$ a month off this site.

imo your best plan would be to take what youve learned from this site as a lesson, pack it up, get to brainstorming till you have a really good idea, and spend your time and hosting moneys on a sick new idea.