Custom branding in SharePoint portal

MeShare115 Here are my few suggestions you can try:

1. Try to use SharePoint OOTB web parts as much as you can. They are responsive by default.
2. SharePoint supports saving pages as templates. So for faster setup, you can use page templates in SharePoint.
3. If you are developing any custom solution using SharePoint framework (SPFx), follow SharePoint grid and responsive design documentation.
4. Also, have a "look" at SharePoint look book to get ideas how you can create & setup beautiful SharePoint sites.

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