I would like to make the forum a bit narrower on all platforms and templates. Is there instructions on how to achieve this?
Go to:
admin-->themes and layout--->theme settings
Change the forum width from 90% to something like 1366px or 1440px or 1600px
Only catch with that is it won't scale for width on narrower screens. Best option is to also adjust max-width in index.css, so that it can't exceed 100%. Personally I'd reduce the min-width too. Something like:
/* This division wraps the entire forum when a forum width is set. */
div#wrapper
{
margin: 0 auto;
min-width: 480px;
max-width: 100%;
}