Design Manager feature in SharePoint 2013
Design Manager is a feature in SharePoint 2013, which will make designer job easier to create a fully customized design while using the web-design tools that we're already conversant with.
Design Manager is a publishing feature that is available in publishing sites in both SharePoint Server 2013 and Office 365. You can also use Design Manager to brand the public-facing website in Office 365. To enable the Design Manager for the SharePoint sites, go to site settings under the Site Collection Feature enable the SharePoint Server Publishing Infrastructure.
Than go to the Site Features section and activate the SharePoint Server Publishing feature.
After activating the features, when click on the Site Settings menu you can the Design Manager has been enable for the designer. Please refer the following screen capture.
With the help of Design Manager, you can create a visual design for your website by using whatever web design tool design prefer, using only HTML and CSS, and then upload that design into SharePoint. Design Manager is the central hub and interface where you manage all aspects of a custom design.