Hi @Mahadev Prasad ,
To brand and customize modern site in SharePoint, please refer to below articles:
The best practice to I think to customize modern UI is to inject JS and CSS files into a SharePoint Modern page is using SPFx Application Customizer:
- Build your first SharePoint Framework Extension
- Inject Custom CSS on SharePoint Modern Pages using SPFx Application Extensions
- Design of Modern Pages in SP Online using CSS/JS injection
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