The viewer supports many visual themes, selected through options. This example applies a theme to the React viewer component.

Viewer setup. Configure the viewer, load the report, assign it to the component, and process the requests or events raised by the UI.
import { Viewer, Stimulsoft } from 'stimulsoft-reports-js-react/viewer';

function App() {
    var report = new Stimulsoft.Report.StiReport();
    report.loadFile('Reports/SimpleList.mrt');

    var viewerOptions = new Stimulsoft.Viewer.StiViewerOptions();
    viewerOptions.appearance.theme = Stimulsoft.Viewer.StiViewerTheme.Office2022WhiteViolet;
    viewerOptions.appearance.fullScreenMode = true;

    return <Viewer options={viewerOptions} report={report} />;
}
The theme comes from the StiViewerTheme enumeration; dozens of light and dark variants are available. Combined with fullScreenMode, the viewer matches the look of your React application.