added custom theming

src/styles/themes/_bootstrap_variables.preview-release.scss

+$gray-100: #e8ebf3 !default; // #eee
+$gray-800: #444444 !default; // #444
+
+$navbar-dark-color: #FFFFFF;
+
+/* Reassign color vars to semantic color scheme */
+$blue: #43515f !default;
+$green: #92C642 !default;
+$cyan: #56ab9c !default;
+$yellow: #ec9433 !default;
+$red: #CF4444 !default;
+$dark: #43515f !default;
+
+$body-color: $gray-800 !default;
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