Please follow the below instructions to change font size and color for 2D label:
In the label properties, copy the class name. You can use the default class-name or define a new name. If you choose the default class-name, it will be applied to all the labels in the application. If you want to change the styles for only a specific label, then use the unique class name for that label.
In the Project pain, expand the Styles option and Select Application.
This will open an editor view where you can enter the CSS code for your experience.