bbPress.org

These two Custom CSS rules should help.

put them anywhere you can put custom CSS

child themes style.css file
jetpacks custom CSS module
a standalone custom css plugin.

.bbpress .post-section {
  margin: 0 2% 20px 2%;
  width: 96%;
}

.bbpress #content-main {
  width: 100%;
  margin-right: 0;
}

if this does not work try this CSS for a test

if the links in your forum turn red the CSS is working.

.bbpress a {
  color: red;
}

if the links are red and my above CSS rules for full width arent working i probably have to redo the CSS rules i gave you above to include more div classes which will be super annoying.