When adding theme.json to an existing Classic Theme, these settings will stop the default color and typography controls from being enabled.

theme.json

{
	"$schema": "http://schemas.wp.org/trunk/theme.json",
	"version": 2,
	"settings": {
		"layout": {
			"contentSize": "750px"
		},
		"color": {
			"background": false,
			"custom": false,
			"customDuotone": false,
			"customGradient": false,
			"defaultGradients": false,
			"defaultPalette": false,
			"text": false
		},
		"typography": {
			"customFontSize": false,
			"dropCap": false,
			"fontStyle": false,
			"fontWeight": false,
			"letterSpacing": false,
			"lineHeight": false,
			"textDecoration": false,
			"textTransform": false
		}
	}
}