Add experimental high contrast mode (#1845)

Add an experimental high contrast color theme to SCM-Manager. The high contrast mode uses a dark background and color with a high contrast for a better accessibility.
The change adds the theme to ui-styles and theme switcher to the storybook of ui-components.

scm-ui/ui-styles/src/variables/_commons.scss

@@ -0,0 +1,34 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020-present Cloudogu GmbH and Contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+//$link: #007EB0;
+$info: #33b2e8;
+$turquoise: #00d1df;
+$blue: #33b2e8;
+$cyan: $blue;
+$green: #00c79b;
+$warning: #ffdd57;
+$blue-light: #98d8f3;
+$danger: #ff3860;
+
+$family-monospace: "Courier New", Monaco, Menlo, "Ubuntu Mono", "source-code-pro", monospace;