Layout – this file contains detailed information about the layout of your report (size, position and settings for every widgets). You can click Download to save off a copy of your dashboard layout at any point (i.e. to take backups). You can then use the Upload button if you need to roll back to a saved layout for example, or to apply a saved layout to a different dashboard.
Colour Scheme – this file contains all the settings you can choose under the Style tab in Dashboard Builder. Again, it can be useful to download/upload the colour scheme for the same reasons given above for the layout.
Stylesheet – use this button to initially download the default stylesheet file for your dashboard. You can then edit this file using a text editor and upload it using the Custom Stylesheet, Upload button in order to apply the custom CSS to your dashboard.
Custom stylesheet – you can use these buttons to download/upload the custom stylesheet for your dashboard. It is worth noting that if you upload a custom stylesheet to your dashboard, subsequent chages made under the Style tab will have no effect, as the custom stylesheet overrides these settings.