This will cause the forum to stretch to the full width of your browser window. If this is too big I've also done a 90% and 75%.
If you want a different width, just edit the style through Stylus and change the percentage, it is very easy!
This can be installed along with one of the Declutter styles.
NOTE: MSE are changing the forum quite a lot at the moment so expect this style to break if they do anything. If something seems wrong please disable all styles first to check.
Notes
v1.0.0
First version
Source code
/* ==UserStyle==
@name MoneySavingExpert.com Forums - 100% Width
@namespace forums.moneysavingexpert.com
@version 1.0.0
@description This will cause the forum to stretch to the full width of your browser window.
@author f300_
==/UserStyle== */
@-moz-document domain("forums.moneysavingexpert.com") {
.Container {
max-width: 100% !important;
}
}