It is not possible.
Depending on the styles changes, you will have to turn those changes into a custom CSS file (and maybe JS file) that you load in the original Simple XML dashboard.
In SimpleXML dashboards, you can extend them by updating the
<form> element at the top of the file. For example, the following will load the two JS files and the once CSS file any time the dashboard is loaded.
<form script="custom.js,custom2.js" stylesheet="custom.css">
For an application, including files "dashboard.css" and "dashboard.js" in the directory "appserver/static/" of your app will result in these two files being automatically loaded for any SimpleXML dashboards in your app.
To create an HTML version of the dashboard, click Create New. Or, to replace and write over the existing dashboard, click Replace Current.
Note Replacing the current dashboard makes the HTML version of the dashboard visible to users and hides the Simple XML version. You can revert to the Simple XML dashboard by deleting the HTML file. The file locations depend on the permissions (see below).
The location of the HTML file depends on the permissions you chose for the dashboard: