I am having problems trying to change the font size to a single survey. I have created a new style sheet and copied the relevant CSS from the core.css file and edited this and saved it as a seperate css file to fit my needs then attached this new css file
into the relevant default.master of my survey page, but it doesnt seem to be making any changes at all.
When you add your reference of your custom css file on your default.master , you should approve your master page.(Site Actions->Site Settings->Modify All site settings->Galleries->Master pages and page layouts). You can add your css file .I suggest
you to use SharePoint designer to custom your self css style.
About how to use SharePoint designer to custom css style, you can refer to the following link.