updated documentation for styles.

Author: bikrant

demo/src/assets/docs/extensions/fixedcolumns/installation.md

@@ -5,8 +5,6 @@ You need to install its dependencies:
 ```bash
 # JS file
 npm install datatables.net-fixedcolumns --save
-# CSS file
-npm install datatables.net-fixedcolumns-bs4 --save
 ```
 
 ##### angular.json
@@ -22,7 +20,6 @@ Add the dependencies in the scripts and styles attributes:
           "options": {
             "styles": [
               ...
-              "node_modules/datatables.net-fixedcolumns-bs4/css/fixedColumns.bootstrap4.css"
             ],
             "scripts": [
               ...
@@ -31,3 +28,43 @@ Add the dependencies in the scripts and styles attributes:
             ...
 }
 ```
+
+#### Update CSS
+
+Update your global style ( genreally styles.css ) as
+
+ ```css
+    /** Fixed columns css 
+   
+   These classes are injected by fixed columns extensions
+   and can be tweaked here to match the colors of headers and body
+   to hide the scrolling element behind the fixed header.
+   
+   */
+   
+   table.dataTable thead tr > .dtfc-fixed-left,
+   table.dataTable thead tr > .dtfc-fixed-right,
+   table.dataTable tfoot tr > .dtfc-fixed-left,
+   table.dataTable tfoot tr > .dtfc-fixed-right {
+     top: 0;
+     bottom: 0;
+     z-index: 3;
+     background-color: white;
+   }
+   
+   table.dataTable tbody tr > .dtfc-fixed-left,
+   table.dataTable tbody tr > .dtfc-fixed-right {
+     z-index: 1;
+     background-color: white;
+   }
+   
+   div.dtfc-left-top-blocker,
+   div.dtfc-right-top-blocker {
+     background-color: white;
+   }
+ ```
+ Alternative to writing css to global file, you can also install a supported css file for this extension from npm library and update it inside ``styles`` property in angular.json.
+
+```bash
+npm install datatables.net-fixedcolumns-bs4 --save
+```