[FIX] website_quote: signature color

When signing an online quotation the pen color depended on the theme
that was installed (white when theme graphene was installed).

addons/website_quote/static/src/js/website_quotation.js

@@ -50,7 +50,7 @@
                 'submit #accept': 'submitForm',
             },
             initSignature: function(ev){
-                this.$("#signature").empty().jSignature({'decor-color' : '#D1D0CE'});
+                this.$("#signature").empty().jSignature({'decor-color' : '#D1D0CE', 'color': '#000', 'background-color': '#fff'});
                 this.empty_sign = this.$("#signature").jSignature("getData",'image');
             },
             clearSignature: function(ev){