use proper variable for ide updater dialog

Author: francescospissu

arduino-ide-extension/src/browser/style/ide-updater-dialog.css

@@ -27,8 +27,8 @@
 }
 
 .ide-updater-dialog .changelog-container {
-  color: var(--theia-dropdown-foreground);
-  background-color: var(--theia-dropdown-background);
+  color: var(--theia-editor-foreground);
+  background-color: var(--theia-editor-background);
   border: 1px solid var(--theia-tree-indentGuidesStroke);
   border-radius: 2px;
   font-size: 12px;