Add dark mode theme

_sass/color_schemes/dark.scss

+
+$body-background-color: $grey-dk-300;
+$sidebar-color: $grey-dk-300;
+$border-color: $grey-dk-200;
+
+$body-text-color: $grey-lt-300;
+$body-heading-color: $grey-lt-000;
+$nav-child-link-color: $grey-dk-000;
+
+$link-color: lighten($purple-000, 8%);
+$btn-primary-color: $purple-200;
+
+$code-background-color: $grey-dk-250;