When migrating to SharePoint 2013 older assets may need serious modification to gain from the benefits of the new, lightweight, fast, and fluid user interface. These benefits come from new CSS styles, themes, and master pages.
Thus, you must re-create your custom branding by using the new styles, themes, or master pages available in SharePoint 2013, and then apply the newly re-created design to the upgraded site collection.
You can read more about the details from a Microsoft support article which outlines the approach for migrating custom UI artifacts to SP2013.
Some of the common approaches suggest creating an evaluation site collection and then, making specific modifications depending on the artifact:
- Custom CSS – use that site as the environment where you can identify the new SharePoint 2013 styles that you need to override. Create a new CSS file for these styles, and then apply that CSS to your upgraded site.
- Custom theme – re-create the theme by using the new theming features in SharePoint 2013.
- Master pages – re-create the master page in the SharePoint 2013 site. After you verify that the new master page works as expected, move the master page to the new site collection and apply it to the site.
- Custom content placeholders on a custom master page – create an evaluation site collection that is also a publishing site, and then set the master page to the out-of-the-box SharePoint 2013 master page. If the site still renders, you don’t have this issue.
Microsoft recommends that you do not add custom content placeholders to your custom master page or page layouts.
In conclusion, I hope this helps with your planning when considering either new initiatives on older releases of SharePoint (i.e. SP 2010), or when migrating to SP 2013.