Added modern browser warning for theme support

postcss.config.js

@@ -0,0 +1,15 @@
+module.exports = {
+    plugins: [
+        require("postcss-import"),
+        require("autoprefixer")({
+            browsers: [
+                "Chrome >= 40",
+                "Firefox >= 35",
+                "Edge >= 14"
+            ]
+        }),
+        require("postcss-css-variables")({
+            preserve: true
+        }),
+    ]
+};

src/web/html/index.html

@@ -223,7 +223,7 @@
                                 <option value="classic">Classic</option>
                                 <option value="dark">Dark</option>
                             </select>
-                            <label for="theme"> Theme </label>
+                            <label for="theme"> Theme (only supported in modern browsers)</label>
                         </div>
                         <div class="option-item">
                             <input type="checkbox" option="update_url" id="update_url" checked />

src/web/stylesheets/index.css

@@ -0,0 +1,34 @@
+/**
+ * CyberChef styles
+ *
+ * @author n1474335 [n1474335@gmail.com]
+ * @copyright Crown Copyright 2017
+ * @license Apache-2.0
+ */
+
+/* Themes */
+@import "./themes/_classic.css";
+@import "./themes/_dark.css";
+
+/* Utilities */
+@import "./utils/_overrides.css";
+@import "./utils/_general.css";
+
+/* Preloader styles */
+@import "./preloader.css";
+
+/* Components */
+@import "./components/_alert.css";
+@import "./components/_button.css";
+@import "./components/_list.css";
+@import "./components/_operation.css";
+@import "./components/_pane.css";
+
+/* Layout */
+@import "./layout/_banner.css";
+@import "./layout/_controls.css";
+@import "./layout/_io.css";
+@import "./layout/_modals.css";
+@import "./layout/_operations.css";
+@import "./layout/_recipe.css";
+@import "./layout/_structure.css";