How to make certain pages narrower (not 1170px full width)?

[startover909]

Using the Six theme. WHMCS defaults to Bootstrap's max 1170px main container width on all pages that do not have a side bar. 1170px is way too wide for pages such as announcements and ticket forms (the readability of a heavy text page suffers once a paragraph exceeds a certain width, generally around 800px). 

So on all these pages, WHMCS wraps them in a  full-width Bootstrap container with the class "col-xs-12 main-content". It would be so much easier if it adds an additional class to identify the page, such as "home-page", "submit-ticket", "announcements" etc like all Wordpress pages do, so we can style them directly in CSS via "max-width" and "margin:auto"  as needed. But WHMCS doesn't do this, and I can't find the actual "col-xs-12 main-content" container in the .TPL template files (the contents in the templates start AFTER the main container). So how do I efficiently limit the width of these pages?

I can use CSS to limit the width of the main element of the page, such as the ticket form, the article containers etc . The problem is that the "header-lined" class container which includes the main heading of the page and the breadcrumbs sit outside the page's contents, right under the "col-xs-12 main-content" container. Again these "header-lined" containers do not have any page identification class added. So these will remain left aligned if we center align the narrowed-down content container on the page.

Your advice would be greatly appreciated.

Edited May 5, 2018 by startover909

[startover909]

I just realized WHMCS apparently does not have any concept on the proper design for text-heavy pages. This community itself is full blown full-width on all pages and posts...

Edited May 5, 2018 by startover909