Created Themes (markdown)

Themes.md

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+PHPNuxBill Support Themes Modification
+
+PHPNuxBill using [Smarty](https://www.smarty.net/) for Interface.   
+Themes saved at **ui/themes**
+
+Folder name use underscore for space, example **ui/themes/my_theme** it will show as **My Theme**
+
+User can Customize Theme or default Theme using **ui_custom**
+
+The priority for the folder are
+
+1. ui/ui_custom
+2. ui/themes
+3. ui/ui
+
+You can create theme for Customer only, Admin Only, or both.   
+But Customer only is recomended.
+
+## Variable
+
+**{$_theme}** is used for path   
+example:   
+```html
+<link rel="stylesheet" href="{$_theme}/styles/bootstrap.min.css">
+```
+it will change to
+```html
+<link rel="stylesheet" href="ui/themes/my_theme/styles/bootstrap.min.css">
+```
+
+**{$_c}** are config variable, key values from table **tbl_appconfig**
+