believe me: I spent days in research to fix my problem but could not find a solution yet...
I use a lot of Excel Web Parts on my SharePoint Site. Basically to display charts or tables. Everything worked fine for months but out of a sudden some of the web parts do not anymore. These Web Parts still get displayed but all in black or grey and because of that are partly unreadable. For example: a cell with a white font and a blue blackground just gets displayed as a black cell with black fontcolour. I thought it might depends on the settings of the server computer but other colourful charts/tables still work fine (but on the other hand, all new web parts do not get displayed correctly...)
I tried to remove macros, conditional formatting, creating completely new subsites with content. But the result is always the same: black cells.
Then I tried to just use 8bit-colours and tada: it worked (except of the colour white, that still does not work).. I do not have access to the server computer. And I am wondering because some other charts still work..
I do not use SharePoint Designer, just the browser version of 2010.
Thanks for your help!!!