The problem come from the class "block-region" is added in the "#region-content" div only when the edit mode is activated in "my page".

That was not the case in the past. This is broke the display of all theme but is more visible in some themes.

To reproduce:

• As admin, select a theme using background-image for the header (Like Splash)
• Go to my page and click on "Customise this page"
• The blocks in the middle will be stylized like the ones on the side who is wrong