Update _default_colors.scss

Add !default flag to allow Sass variable overrides
This allows a user of the theme to customize the value of a variable
defined in variables.scss by defining it before the `@import` statement:

```scss
---
---

$section-headings-color: #0086b3;

@import "{{ site.theme }}";
```

_sass/_default_colors.scss

@@ -1,15 +1,15 @@
-$apple-blossom: #ac4142;
-$alto: #d0d0d0;
-$bouquet: #aa759f;
-$chelsea-cucumber: #90a959;
-$cod-grey: #151515;
-$conifer: #b5e853;
-$dove-grey: #666;
-$gallery: #eaeaea;
-$grey: #888;
-$gulf-stream: #75b5aa;
-$hippie-blue: #6a9fb5;
-$potters-clay: #8f5536;
-$rajah: #f4bf75;
-$raw-sienna: #d28445;
-$silver-chalice: #aaa;
+$apple-blossom: #ac4142 !default;
+$alto: #d0d0d0 !default;
+$bouquet: #aa759f !default;
+$chelsea-cucumber: #90a959 !default;
+$cod-grey: #151515 !default;
+$conifer: #b5e853 !default;
+$dove-grey: #666 !default;
+$gallery: #eaeaea !default;
+$grey: #888 !default;
+$gulf-stream: #75b5aa !default;
+$hippie-blue: #6a9fb5 !default;
+$potters-clay: #8f5536 !default;
+$rajah: #f4bf75 !default;
+$raw-sienna: #d28445 !default;
+$silver-chalice: #aaa !default;