body {
   background: none;
}
@media (min-width: 700px){
   .post-inner {
       padding-top: 0;
   }
}

The CSS codes should be added in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Style Tab.

Please note, all questions related to your active theme should be asked in the theme's support forum, not here. 

The issue comes from the active theme, it sets the following CSS rule: 

body.template-full-width .entry-content > *:not(.alignwide):not(.alignfull):not(.alignleft):not(.alignright):not(.is-style-wide), body.template-full-width  > *:not(.alignwide):not(.alignfull):not(.alignleft):not(.alignright):not(.is-style-wide) {
      max-width: 120rem;
}

If you want to overwrite that rule use the CSS code below:

body.template-full-width .entry-content > *:not(.alignwide):not(.alignfull):not(.alignleft):not(.alignright):not(.is-style-wide) {
   width: 100% !important;
   max-width: 99%;
}

The CSS codes should be added in the "Custom CSS code" textarea, located in the Dashboard > Forums > Settings > Style Tab.  

The red marked codes can be changed as you like. 

Please don't forget to delete all caches and press Ctrl+F5 (twice) on the frontend before checking to reload the updated CSS file.