1. Log into AdminCP
2. Navigate to Styles & Templates > Your style > StyleVars
3. In the search field type in doc_width (under Common).
4. Now set the Units and Size as per your requirement, and Save. (default: auto)
That's it!
Now if at all for some reason your theme is not center aligned after adjusting the width do this:
•Type doc_margin in the search field.
•Set Left & Right to auto
Now you have your CMS, Blog and Forum center aligned to the width of your choice!
https://vborg.vbsupport.ru/showthread.php?t=231671