fix for install button in libs and board manager

Author: francescospissu

arduino-ide-extension/src/browser/style/list-widget.css

@@ -145,3 +145,14 @@ https://github.com/arduino/arduino-pro-ide/issues/82 */
 .component-list-item a:hover {
     text-decoration: underline;
 }
+
+/* High Contrast Theme rules */
+/* TODO: Remove it when the Theia version is upgraded to 1.27.0 and use Theia APIs to implement it*/
+.hc-black.hc-theia.theia-hc .component-list-item .header .installed:hover:before {
+    background-color: transparent;
+    outline: 1px dashed var(--theia-focusBorder);    
+}
+
+.hc-black.hc-theia.theia-hc .component-list-item .header .installed:before {
+    border: 1px solid var(--theia-button-border);
+}