@jalin0919

This is a theme related issue. You can use the CSS code below to solve it:

.main-wrap .container {
    max-width: 97%;
    padding-right: 0;
    padding-left: 0;
}

The red marked value can be changed as you like. 

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

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

Also, please try on incognito mode if you're checking on mobile devices. Because mobile browsers have a hard cache, you'll not be able to see the change immediately.

Thanks for your support.

I resolved problem with below code add to Custom CSS of Forum page.

@media screen and (max-width: 700px){
.main-wrap .container {
padding-right: 15px;
padding-left: 15px;
}