Search and Replace is not appropriate for use in "Design" view insofar as changing CSS. If you highlight text in Design view, you'll notice on the Tools Bar what CSS is applied to that area. There are 3 drop down menus for this: Type of area, e.g. Paragraph, H1, H2, etc, Font Family, and Font Size. You can change your styles using these drop downs.
You can also display the CSS using the CSS Properties window. Click on "Task Panes" then select CSS Properties. It will probably appear below your "Folders" pane, but you can drag/move it with your cursor to where ever is comfortable.
Your other choice is go into Code View and find the specific CSS you are looking for in the <head> then search for it using the find feature. You can also use the split view which will show you the applicable code as you highlight the text.
Hope this helps,