How to change the style of a markup in VSW?

You can accomplish this by utilizing the custom command: sketching.edit-geometry. Below is an example of how to edit existing markups in the VSW Capital City Sample App:
 

1. Navigate to the area in VSW where want configure the functionality and configure a new Custom Command:

2. Now, you can add the following command, f.e.:

[
    {
        "name": "sketching.capture-geometry",
        "arguments": {
            "geometryType": [
                "point",
                "extent"
            ]
        }
    },
    "map.get-markup",
    {
        "name": "sketching.edit-geometry",
        "arguments": {
            "pluginSettings": {
                "snapping": true
            },
            "symbol": {
                "type": "esriSFS",
                "color": [
                    150,
                    150,
                    150,
                    0
                ],
                "style": "esriSFSSolid",
                "outline": {
                    "color": [
                        14,
                        94,
                        231,
                        255
                    ],
                    "width": 1.5,
                    "style": "esriSLSSolid",
                    "type": "esriSLS"
                }
            }
        }
    }
]

 3. The GIF shows you the edit markup command in action, which is configured in the toolbar:

Attached is an example app that includes this command.