fix upload certificate dialog background

Author: francescospissu

arduino-ide-extension/src/browser/style/certificate-uploader-dialog.css

@@ -71,4 +71,10 @@
   width: 100%;
   box-sizing: border-box;
 
-}
+}
+
+/* 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 .certificate-add {
+  background-color: var(--theia-editorWidget-background);
+}