How I can brand search results page in SharePoint Foundation 2013? I have brand other sites using custom.css file, however it dosen't work in search result page at all. What am I missing here? How I can change background etc?
The search results page exist in a search template/web part. You can edit these and even create your own based on the standard templates.
To see all the default search display templates,
go to Site settings --> Master pages and page layouts. In the Master Page Gallery, click Display Templates --> Search.
The following diagram is how it all hangs together.
The following blog post explains it all in full detail. The 2nd link is how to specifically create your own.