Hello Abhinav,
I would suggest starting from the basic OutSystems UI and just update the branding colors (primary/secondary) and logo. It's easier to go over the existing UI StyleGuide and try to update the CSS theme to match your branding colors than importing another theme into OutSystems.
You can use this Forge component to preview your changes to the theme as you go: https://www.outsystems.com/forge/component-overview/7527/outsystems-ui-style-guide-preview
Regards,
Bogdan