Help please – blog colours gone crazy

Been using 2.7 and the Mandigo 1.39 theme for a fortnight with no problems at my blog :

http://www.caledonian-comment.com

Then all of a sudden today, problems. Blog header colours all over the place, with double and treble presentations superimposed. Text coilours not as they should be. Then incredibly slow to access website. Problem started with Firefox, then spread to IE.

Changed theme to Mandigo 1.40 and problems still persist. Any ideas please ? Thanks.