fix(datePicker): ensure one is able to override default sass variables values

Closes #7329

Author: sryzycki

src/components/datepicker/datePicker.scss

@@ -1,8 +1,8 @@
 /** Styles for mdDatepicker. */
-$md-datepicker-button-gap: 12px;  // Space between the text input and the calendar-icon button.
-$md-datepicker-border-bottom-gap: 5px;  // Space between input and the grey underline.
-$md-datepicker-open-animation-duration: 0.2s;
-$md-datepicker-triangle-button-width: 36px;
+$md-datepicker-button-gap: 12px !default;  // Space between the text input and the calendar-icon button.
+$md-datepicker-border-bottom-gap: 5px !default;  // Space between input and the grey underline.
+$md-datepicker-open-animation-duration: 0.2s !default;
+$md-datepicker-triangle-button-width: 36px !default;
 
 md-datepicker {
   // Don't let linebreaks happen between the open icon-button and the input.