Hi Marekmak,
To change the background color you have to upgrade to the premium version of WP Portfolio theme where you can have a lots of features with color option to change the specific section. However, you can change it via custom css to set the background go to Appearance -> Customize -> WP Portfolio Theme Options -> Custom CSS and paste the below CSS code then click on Save button.
#page:before, .entry-main {
background-color: #fff;
}
Change above HEX color code #fff to you own HEX color code.
Also we have spend lots of time and efforts to develop this theme available free for all.
Could you also please help us rating this theme in WordPress Repository.
https://wordpress.org/support/view/theme-reviews/wp-portfolio
We really appreciate your rating.
Thank you!