Don't know who's interested but this is my current css tweaks (goes in ublock origin's "my filters")
Code:
!right sidebar
www.racedepartment.com##.p-sidebarWrapper
!left sidebar
www.racedepartment.com##.p-body-sideNavInner
www.racedepartment.com##.p-body-sideNavCol:style(width:0px !important)
www.racedepartment.com##.p-body-sideNav:style(width:0px !important)
!fill main body out to width freed up from sidebars
www.racedepartment.com##.p-body-content:style(max-width:100% !important)
! revert to user info next to posts
www.racedepartment.com##.p-body-main:style(display:block !important)
www.racedepartment.com##.avatar--m:style(width:96px !important; height:96px !important )
www.racedepartment.com##.avatar:style(border-radius:0% !important)
www.racedepartment.com##.message-inner:style(display:flex !important)
www.racedepartment.com##.message-cell--user > section:style(display:block !important)
! remove stuff below bottom post
www.racedepartment.com##.shareButtons.shareButtons--iconic
www.racedepartment.com##[data-widget-id="88"]
Removes both sidebars on the forums, the stuff that was in between the bottom post & the quick reply, moves avatars back to the side of the post, makes them square instead of round, makes the 'medium' avatars bigger since they're not stretching the layout vertically, all in all it makes the forum only a forum, instead of 80% static content I'm not going to click.
no warrantee etc. etc. but if you don't like it you can just delete it again. The stuff below the bottom post was completely breaking the site so you probably can't delete that by itself (I think related to p-body-content's max-width 100%), but if you like other changes you can delete those sections.
I tried the 'change width' button and it doesn't seem to do anything at my window size