CLN: Styler Types for CSS variables on `ctx` object. 
- [x] tests changed / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

`Styler` currently has three formats for keeping track of CSS:

 A) `CSSList type: [('a1', 'v1'), ('a2', 'v2')]`
 B) `List[str] type: ['a1:v1', 'a2:v2']`
 C) `Str type: 'a1:v1; a2:v2;'`

`C` is only ever used as user input format, and now, as of #39564 `C` can be used in *all* cases for user input (albeit `A` remains available for backwards compatibility in `.set_table_styles()`)

`A` remains the most algorithimcally accessible, and is what `C` is already converted to in some cases for internal variables.

`B` is only ever used by one internal variable. The `ctx` object is updated with `B` type converted from `C`. For final render this `B` type is converted to `A` type for looping. In the process, white space is poorly handled.

This PR eliminates `B` type and converts `C` directly to `A`. 

Tests using the `ctx` object needed updating for new type.
