CSS color property represents color of text. If element has no text, the color property does nothing.
Slider has text but not directly, it has text inside the bubble. Adding color property to the slider element, does not change the color value of the text inside the bubble.
bgcolor is not valid CSS property. There is background-color property which represents color of the elements background.
But i think you mean the configuration options, it is limited as every node can do and what is reasonable to provide. It is not easy to make all options for all possible elements to be configurable. If needed to be customized, it takes to make overrides for CSS.
Can you imagine the size of the configuration page where all elements which make the widget are fully described by every option they have? And also if all options are configurable that easily, how it would look like? And where to draw the line what should and what should not to be available for configuration?
The target of the dashboard is to provide option to build it up quickly and without need to have any knowledge of front-end coding. The look is so to say standard so it allows to achieve it mostly with default settings. Everything above is already custom and as the sky is limit - there is no way to draw the line between what is reasonable and what is not.
But it does not end here as with the ui_template node you can define styles for all dashboard elements. There is a lot of threads and examples about customizing dashboard CSS. Just take a time and do search in therms dashboard CSS ...