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Unfortunately you can't at this time. Because the widget is loaded on a separate iframe, you cannot style it with the CSS code on the form only.
It could be possible if the Slider Widget have its own custom CSS property also where you can inject CSS code to customize the style/color of the text.
Do you mean to say that you want to change the background color of the "Sliders with calculated result" widget?
As my colleague explained, it is currently not possible to inject custom css code to change the appearance of the slider with calculated result widget. We have already sent a feature request ticket to our back end team. We will get back to you as soon as we have any update form them.