Change the css rule of 

elgg-layout-one-sidebar {
background: transparent url(<?php echo elgg_get_site_url();?>_graphics/sidebar_background.gif) repeat-y right top;
} 

See http://docs.elgg.org/wiki/Themes

But how do I change it I have tried everything from editing the image to switching the image for a color, but when I change it to a color it messes the entire page up.

That css rule can't mess up your page, you are doing something wrong. You can change the background to a color, change the color of the graphic or even delete the rule;

.elgg-layout-one-sidebar {
    background: #F0F0F0;
}

Try http://community.elgg.org/plugins/839775/1.8.3/customize-css

When I do that nothing changes, is it because I am using river activity 3c?

css gets cached - if you haven't tried clearing your caches that's the most likely reason

Just go to administration panel, Advanced Settings and turn off,

Use simpel cache and use system cache

Then you can see your edits right away.

I use upgrade.php everytime but nothing I have no idea what is going on, sorry to be a bother

Iam not sure my answer is correct..you go to your installed theam(in mod directory) and find the web galli said css chage it

go to administration flush the caches and run upgrade.then you check and clear your browser history also... :)

Keep your theme at the bottom of the plugin list.

3C river activity does not over ride any of the sidebar CSS.... Only CSS it over ride is the CSS for pop up module. that too it set a margin to the bottom of each popup box. thats all.