Improve support for dark theme

Author: zbynek

src/main/resources/io/jenkins/plugins/bootstrap5.jelly

@@ -15,6 +15,7 @@ Use it like <st:adjunct includes="io.jenkins.plugins.bootstrap5"/>
   <script type="text/javascript" src="${resURL}/plugin/bootstrap5-api/js/popper.min.js"/>
   <script type="text/javascript" src="${resURL}/plugin/bootstrap5-api/js/bootstrap.min.js"/>
   <script type="text/javascript" src="${resURL}/plugin/bootstrap5-api/js/no-prototype.js"/>
+  <script type="text/javascript" src="${resURL}/plugin/bootstrap5-api/js/theme-support.js"/>
 
   <script>
     const bootstrap5 = bootstrap;

src/main/webapp/js/theme-support.js

@@ -0,0 +1,10 @@
+if (window.getThemeManagerProperty) {
+    const setBsTheme = () => document.body.dataset['bsTheme'] = getThemeManagerProperty('bootstrap', 'theme');
+    setBsTheme();
+
+    if (window.isSystemRespectingTheme) {
+        window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', event => {
+            setBsTheme()
+        });
+    }
+}