docs(3rd-party): add docs on 3rd party styles

docs/articles/concept-3rd-party.md

@@ -1 +1,34 @@
-Documentation is being developed.
+Currently Nebular Theme system is integrated with the following 3rd party modules:
+
+- Bootstrap 4;
+- ng2-smart-table;
+- angular2-toaster.
+
+That mean that we created style overriding for these modules, so that you can change component's look & feel using Nebular theme variables.
+For example, if you change the `color-primary` variable, bootstrap components using this color will be changed accordingly.
+
+
+Or that's how we described variables for the `angular2-toaster` module:
+
+```scss
+  ...
+  toaster-bg: color-primary,
+  toaster-fg-default: color-inverse,
+  toaster-btn-close-bg: transparent,
+  toaster-btn-close-fg: toaster-fg-default,
+  toaster-shadow: shadow,
+
+  toaster-fg: color-white,
+  toaster-success: color-success,
+  toaster-info: color-info,
+  toaster-warning: color-warning,
+  toaster-wait: color-primary,
+  toaster-error: color-danger,
+  ...
+
+```
+
+## Next
+
+- [Enable Theme System](#/docs/guides/enabling-theme-system).
+- [Theme System Concepts](#/docs/concepts/theme-system).

docs/articles/concept-ui-kit.md

@@ -1,4 +1,4 @@
-Nebular Ui Kit includes a list of essential component allowing you to quickly create a new page and start filling it with business logic not worrying about essential layout components.
+Nebular Ui Kit includes a list of essential component allowing you to quickly create a new page and start filling it with business logic not worrying about essential layout elements.
 
 Here's a list of Nebular components:
 

docs/articles/enabling-theme-system.md

@@ -148,3 +148,9 @@ enableDarkTheme() {
   this.themeService.changeTheme('dark');
 }
 ```
+
+## Next
+
+- [Theme System Concepts](#/docs/concepts/theme-system).
+- [Default Theme variables table](#/docs/themes/default).
+- [Cosmic Theme variables table](#/docs/themes/cosmic).