update contributing.md to explain updated LESS/CSS customization

CONTRIBUTING.md

@@ -152,6 +152,15 @@ Generate the documentation with:
 
     cd docs && ./build.sh
 
+## CSS Themes
+As part of the npm build process, CSS for Panoramix is compiled from ```Less```, a dynamic stylesheet language.
+
+It's possible to customize or add your own theme to Panoramix, either by overriding CSS rules or preferably
+by modifying the Less variables or files in ```assets/stylesheets/less/```.
+
+The ```variables.less``` and ```bootswatch.less``` files that ship with Panoramix are derived from
+[Bootswatch](https://bootswatch.com) and thus extend Bootstrap. Modify variables in these files directly, or
+swap them out entirely with the equivalent files from other Bootswatch (themes)[https://github.com/thomaspark/bootswatch.git]
 
 ## Pull Request Guidelines
 

panoramix/assets/stylesheets/less/index.less

@@ -1,3 +1,5 @@
+// Index .less, any imports here will be included in the final css build
+
 @import "~bootstrap/less/bootstrap.less";
 @import "./variables.less";
 @import "./bootswatch.less";

panoramix/assets/stylesheets/less/variables.less

@@ -1,4 +1,4 @@
-// Paper 3.3.6
+// Modified from Bootswatch Paper 3.3.6
 // Variables
 // --------------------------------------------------