renamed theme to Mantis

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

-@import url('https://fonts.googleapis.com/css?family=Source+Sans+Pro:200,200i,300,300i,400,400i,600,600i,700,700i,900,900i&subset=cyrillic,cyrillic-ext,greek,greek-ext,latin-ext,vietnamese');
-
-$font-family-sans-serif: 'Source Sans Pro', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
-$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;
-$cyan: #2e80a3 !default;
-$yellow: #ec9433 !default;
-$red: #CF4444 !default;
-$dark: #43515f !default;
-
-$body-color: $gray-800 !default;
\ No newline at end of file

src/styles/themes/_custom_variables.mantis.scss

-$banner-text-background: rgba(0, 0, 0, 0.45);
-$banner-background-gradient-width: 300px;
+$banner-text-background: rgba(0, 0, 0, 0.35);
+$banner-background-gradient-width: 300px;
\ No newline at end of file

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

-$banner-text-background: rgba(0, 0, 0, 0.35);
-$banner-background-gradient-width: 300px;
\ No newline at end of file