What properties are you looking to change exactly?

From this documentation link, I managed to change the background colour of the Intellisense widget.

I added this property to the settings.json file:

"editorSuggestWidget.background": "#fff"

The final file looks like this:

{
    "workbench.colorCustomizations": {
        "editorSuggestWidget.background": "#fff"
    }
}

Not sure if the below is useful to you but:

To access settings.json, I went to File > Settings > Preferences and typed 'color'; scrolled a few items down and found an option to access settings.json. It was created as part of the .vscode folder in my current workspace.