Due the special characteristics of the Basic Search Site our themes are not compatible with it, this type of site just works with the original Master Pages from SharePoint if they are based on the site defenition.
If you apply the theme to this kind of site you will get the error bellow:
How to fix it?
1 - Open you basic search site with SharePoint Designer 2013.
2 – Go to All Files > _catalogs > masterpage.
3 – Select one of the original Master Pages (seattle or oslo).
4 – From the context menu choose Set as Default Master Page and the choose Set as Custom Master Page.
5 – If you have edited the original Master Page you'll also have to Reset the master page to the site definition.
As alternative you can use the Enterprise Search Site that works pretty well with all our themes.