StylableProperty
Properties which can be styled by a theme must use this instead of a Property.
The purpose of a StylableProperty is to hold the value prior to being styled, so that it can be reverted if the property becomes un-styled.
For example, imagine we have a Label with black text, and we add a "highlight" style, which causes the text to turn bright green.
Later, when we remove the "highlight" style, the text will revert to black (using revert).
If a StylableProperty's value is set after it has been styled, the value applied by the Theme will be overwritten. removing the theme will have no effect, the new value will remain.