The content of a page should be inside .page-content element:
<div class="main-container bgc-white">
<div id="sidebar" class="sidebar sidebar-light expandable">
...
</div>
<div class="main-content" role="main">
<div class="page-content">
...
</div>
</div><!-- main -->
</div>
.page-content has some default padding from sides but you can also use custom classes:
<div class="page-content px-2 px-md-3 px-xl-4">
...
</div>
Inside .page-content is optional.
In Ace's demo, I add a .page-header which includes .page-title and .page-tools in some pages.
For example:
<div class="page-header">
<h1 class="page-title text-primary-d2">
Dashboard
<small class="page-info text-secondary-d2">
<i class="fa fa-angle-double-right"></i>
overview & stats
</small>
</h1>
<div class="page-tools">
<button type="button" class="btn btn-sm btn-lighter-default btn-h-lighter-purple btn-a-lighter-purple border-b-2">
<i class="fa fa-save text-120 text-purple-m1"></i>
</button>
<button type="button" class="btn btn-sm btn-lighter-default btn-h-lighter-success btn-a-lighter-success border-b-2">
<i class="fa fa-undo text-110 text-success-m1"></i>
</button>
</div>
</div>
You can change .page-header's border or color by using custom classes such as .text-purple, etc
In Dashboard most content boxes are .ccard elements which has adds a border, a small shadow, rounded corners and white background to the element.
You can also use it with .card element:

<div class="ccard px-3">
My content
</div>
<div class="card ccard">
<div class="card-header">
...
</div>
<div class="card-body">
...
</div>
</div>
There are also .bcard (used in Dashboard 2) and .dcard with some differences.

.acard is a transparent card.
A sample breadcrumbs is like this:

<ol class="breadcrumb breadcrumb-nosep align-items-center pl-1 text-nowrap mr-2">
<li class="breadcrumb-item text-secondary text-100">Forms</li>
<li class="mx-15 text-grey-l4 text-110">/</li><!-- separator -->
<li class="breadcrumb-item active text-dark-tp4 text-105 text-600">Basic Elements</li>
</ol>
.breadcrumb-nosep to .breadcrumb to hide the auto-inserted separator and insert custom separators instead.
You can also have .content-nav element with custom background, border and padding classes, before .page-content:
<div class=" content-nav mb-1 bgc-grey-l4">
<div class="d-flex justify-content-between align-items-center">
<ol class="breadcrumb pl-2">
...
</ol>
<div class="nav-search">
...
</div>
</div>
</div>
<div class="page-content">
...
</div>
See Content Themes