How do I address Properties of an element of the Form if the element has null for a name?

I have begun examining the Properties in the Launcher windows as Shai suggested, and that gives many of the names required to change or apply Styles.

I have found the component text I need to Style on my Multibutton in the Properties (Line1), and was able to Style the text. I changed the background color of the Multibutton and the SwipeableContainer, but as shown in the picture, there is a white rectangle that does not accept the background color change(Gap?).

Since the frame around the label is contained in the Multibutton, I do not understand why the rectangle doesn't accept the background color in the Multibutton Style.

How can I identify and work with the elements without a name?

They are of UIID Container as seen in the screenshot below. Container is a special case UIID which is assumed to be transparent with no padding/margin so you would normally not customize it.

Labels/Buttons are renedered as zero size when they are blank. That means they will have no impact on the UI and don't need customization. So the white color comes from the MultiButton UIID. Just customize that in the CSS and everything should work. Make sure to customize the selected and pressed styles too.

To be clear, you can style the CSS live as the simulator is running and instantly see the impact of your CSS changes without restarting the simulator.