CLN: simplify `Styler._translate`
Bit of a pre cursor to some other cleaning in #43649.

Prevents needing to pass variables around in worker functions, and has the added benefit of making the CSS classes user editable, by storing as a class attribute instead of module constants.

I ran the updated styler ASVs for this and "BENCHMARKS NOT SIGNIFICANTLY CHANGED"
@jreback got this to green. would appreciate pushing it through as a dependency to other prs. Hello @attack68! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-10-15 16:48:05 UTC @jreback ping green @jreback can I bump this up. Other PRs depend on it. Will change if you have strong opinion. ping @jreback. removed both and tested green. just noticed a doc update was also needed now. thanks @attack68  very nice, just a comment for followup.