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/public/js/styleguide.js

@@ -0,0 +1,48 @@
+/*
+ * 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.
+ */
+const rgb2hex = (c) => "#" + c.match(/\d+/g).map((x) => (+x).toString(16).padStart(2, 0)).join``;
+
+function onClickColorButton(e) {
+  const button = e.target;
+
+  const cell = button.parentElement;
+
+  const div = cell.querySelector("div.color-text");
+  if (div) {
+    div.remove();
+  } else {
+    let color = window.getComputedStyle(button).backgroundColor;
+    color = rgb2hex(color);
+
+    const colorText = document.createElement("div");
+    colorText.className = "color-text";
+    colorText.innerText = color;
+
+    cell.appendChild(colorText);
+  }
+}
+
+document.querySelectorAll("table.colors span.button").forEach((button) => {
+  button.addEventListener("click", onClickColorButton);
+});