allow overide of docs variables

Author: m5o

assets/scss/_variables.scss

@@ -1,9 +1,9 @@
 // Local docs variables
-$bd-purple:        #563d7c;
-$bd-purple-bright: lighten(saturate($bd-purple, 5%), 15%);
-$bd-purple-light:  lighten(saturate($bd-purple, 5%), 45%);
-$bd-dark:          #2a2730;
-$bd-download:      #ffe484;
-$bd-info:          #5bc0de;
-$bd-warning:       #f0ad4e;
-$bd-danger:        #d9534f;
+$bd-purple:        #563d7c !default;
+$bd-purple-bright: lighten(saturate($bd-purple, 5%), 15%) !default;
+$bd-purple-light:  lighten(saturate($bd-purple, 5%), 45%) !default;
+$bd-dark:          #2a2730 !default;
+$bd-download:      #ffe484 !default;
+$bd-info:          #5bc0de !default;
+$bd-warning:       #f0ad4e !default;
+$bd-danger:        #d9534f !default;