Looking at your site it looks like you are using the denim theme.
The easiest way to change most things with this theme including the background (especially if you have limited CSS knowledge) is to visit the uikit 2 customizer found here:
https://getuikit.com/v2/docs/customizer.html
By default the denim theme uses the default uikit 2 theme as it's base so select that. You can then change any of the colors and styles and then click on "Get CSS" button when finished.
Then when you have the file replace the following files with the minified version and a non minified version.
\public_html\layout\denim\css_ltr\style.css
\public_html\layout\denim\css_ltr\style.min.css
Once you do this Clear Cache in Geeklog and you should see the changes
You can also change the uikit 2 theme from "default" to "almost flat" or "gradient". This would require you to change the corresponding files found in the directory: \public_html\layout\denim\css_ltr\
You would also be required to update the file \public_html\layout\denim\functions.php and change the line (2hould be #48):
'uikit_theme' => 'default', // you can set this variable to 'default' or 'gradient' or 'almost-flat'
Any other CSS changes should go into the file \public_html\layout\denim\css_ltr\custom.css
If you want to modify the template files that is another post. Ideally you would create a child theme based on the denim theme and only include the changes you want. THis makes updating Geeklog much easier while preserving your theme changes.
One of the Geeklog Core Developers.