Docs improve (#3246)

* Fixed Top nav demo img size

* Added Dark mode Info

docs/components/main-header.md

@@ -296,7 +296,7 @@ Top navbar example can be found in this [demo page](https://adminlte.io/themes/d
   <div class="container">
     <a href="index3.html" class="navbar-brand">
       <img src="{{ '/assets/img/AdminLTELogo.png' | prepend: site.baseurl }}" alt="AdminLTE Logo" class="brand-image img-circle elevation-3"
-           style="opacity: .8">
+           style="opacity: .8; max-height: 33px;">
       <span class="brand-text font-weight-light">AdminLTE 3</span>
     </a>
     <!-- Left navbar links -->

docs/layout.md

@@ -49,6 +49,13 @@ You can also use the following classes for responsive changes with placing
 {: .quote-info}
 
 
+#### Dark Mode
+
+AdminLTE 3.1 provides a dark mode option. You can add in body tag:
+
+- `.dark-mode`
+
+
 #### Color Variations
 
 AdminLTE 3.1 provides a set of color variations to apply to your sidebar (light & dark) & navbar. You can combine any available color with these class prefixes:
@@ -66,6 +73,10 @@ AdminLTE 3.1 provides a set of color variations to apply to your sidebar (light
 > You can combine `.navbar-*` with `.navbar-light` or `.navbar-dark`.
 {: .quote-info}
 
+> ###### Recommended for Dark Mode
+> You must use only `.navbar-dark` without only combination.
+{: .quote-info}
+
 The following colors are available:
 
 ##### Theme Colors