Add custom stylesheet

-The custom.scss file can be used to add custom css changes without affecting the ability to do a git pull update
-Added a quick explanation of using the custom.scss file in the "Configure Explorer Settings" section of the README

scripts/prestart.js

@@ -97,12 +97,18 @@ check_argument_passed(function(retVal) {
   fs.writeFile('./public/css/_theme-selector.scss', `$theme-name: "${settings.shared_pages.theme}";`, function (err) {
     const sass = require('sass');
 
-    // generate minified css from scss file
+    // generate minified css from style.scss file
     const minified = sass.compile('./public/css/style.scss', {style: 'compressed'});
 
     // save the minified css to file
     fs.writeFile('./public/css/style.min.css', minified.css, function (err) {
-      // Finished pre-loading
+      // generate minified css from custom.scss file
+      const custom_minified = sass.compile('./public/css/custom.scss', {style: 'compressed'});
+
+      // save the minified css to file
+      fs.writeFile('./public/css/custom.min.css', custom_minified.css, function (err) {    
+        // Finished pre-loading
+      });
     });
   });
 });