Set theme as @computed value

Signed-off-by: Alex Andreev <alex.andreev.email@gmail.com>

src/renderer/components/select/select.tsx

@@ -38,7 +38,9 @@ export class Select extends React.Component<SelectProps> {
     menuPortalTarget: document.body,
   }
 
-  private theme = this.props.themeName || themeStore.activeTheme.type;
+  @computed get theme() {
+    return this.props.themeName || themeStore.activeTheme.type;
+  }
 
   private styles: Styles = {
     menuPortal: styles => ({