- pass full theme context (matching all our other helpers) instead of just `euiTheme`

- use `is` type predicate instead of `as` cast

- use getters to repeat vars instead of comments (which updates the thumb bg color to match in high contrast mode)

- on focus, can be either the focus color or the level color

- we'll be using this var shortly for higher contrast borders

+ make the outlines of focused thumbs inherit from the level (more polished) in non-high contrast mode as well

…hemes

+ tech debt - fix `any` typing

…k mode visibility

+ tech debt - dogfood new arrow styles helper. this adds another mildly repetitive/annoying Emotion className, but that tradeoff is probably worth it

+ other misc CSS cleanup/simplifications

- minute Loki diffs appear to be coming from aliasing from the transparent border, but shouldn't be a meaningful change

+ add more stories/VRT

…contrast themes

+ add VRT

- by using `aspect-ratio` instead of padding-bottom hack

- remove transparent background - it's more important that the thumb be visible

- remove box shadows and make borders/outlines extra thick

+ always show outlines, on mouse focus, not just keyboard focus

- open to feedback - dark mode feels better *while* focused, but not necessarily while not focused

…ring calendar selections invisible

+ DRY out `euiFormControlDefaultShadow` usage/unsets with newer API

- needs updating still isn't great, but I think this is the best I can do for now :/

…ight & dark modes