text: Title: [Docs] Revise the documentation for layouts to use colours from the palette
  Body: A recent PR introduced a [set of diagrams with very jarring colours](https://vizro.readthedocs.io/en/stable/pages/user-guides/layouts/#custom-layout-examples) that were chosen mostly to contrast strongly and clarify the layout grid. It should be possible to use colours from our standard palette rather the RGBM and this comment includes those we could try.
  
  [Here is the palette](https://github.com/mckinsey/vizro/pull/507#pullrequestreview-2092972824)
  
  [Here is the code in a notebook to create one of the graphics](https://github.com/stichbury/vizro_projects/blob/main/project.ipynb)
  
  The CSS is in the same repo: https://github.com/stichbury/vizro_projects/