You tagged this question as Studio Pro v8. Did you do an upgrade from Mendix 8 Beta 2 to Beta 3 before this issue happened? Because in that case the Atlas UI resources module needs to be updated from the App store, to get the styling in Design mode and Live app working again. (link to release notes)
If not, could you file a support ticket for this? Then we could have a look at it and see if this is maybe caused an issue in the Mendix 8.0 Beta 3 version.
Maybe your page is using a layout that is not picked up by the Design mode. In the page’s properties, under General, change the layout of the page to a modern one like Atlas_TopBar. See if that makes a difference.
Otherwise, try again with a new page.
I re-uploade the Atlas_UI_Resources_2.0.0 and the project in the New uploaded Atlas_IU_Resources file has of cource the version 2.0.0
But when I open my project I got the old version :
now could you please tell me how to replace the old version of the Atlas_UI_Resources with the the new one ? how to update it or where I have to put the extract file from the uploaded Atlas_IU_Rsources exactly?
Please, and thanks !!