refactor: emotion best practices and widget container stacking context

[Charlie-XIAO]

Extracted from and refactored upon #370.

The changes are mostly based on emotion best practices. In particular, define styles outside of components, and use the style prop for dynamic styles.

Another change is to the widget container, which avoid using zIndex: 9999 but rely on relative stacking context (with an additional wrapper layer) to make sure the handle stays above the component.