Dark background

- [*] tests added / passed
- [*] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [*] Color in boxplot.py should be a hex value. Letter based color will be at matplotlib level. Using same color as that of boxes for the caps as well. The color will be fetched based on the selected theme. We are over riding color with "k" , rather need to use the one set at matplotlib level. 

Before Fix : 
> Caps for the box plot is not visible : 
![image](https://user-images.githubusercontent.com/9417467/114264636-ab0bea00-9a09-11eb-943a-5e7cda090ed9.png)

After Fix : 
> Caps for the box plow will be visible in dark background mode : 
![image](https://user-images.githubusercontent.com/9417467/114264600-7009b680-9a09-11eb-870c-5633bd336d1e.png)

Whats New : Box Plot's Caps will have same color as boxes unless color is explicitly specified by user arguments. Hence theme changes will not adversly effect caps color. 


Hello @yeshsurya! 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 2021-05-06 07:17:23 UTC > if your code changes the attribute `_caps_c` to `colors[0]` why is the test still testing the value that the color for `caps` is `k`?

yes , it shouldn't be. I was invoing df.boxplot() in test but I should be testing df.plot.box(). I've changed this.  looks fine if you can add a whatsnew note as @charlesdong1991 suggests and ping on green (bug fix section of 1.3 in visualization) > looks fine if you can add a whatsnew note as @charlesdong1991 suggests and ping on green (bug fix section of 1.3 in visualization)

Added whatsnew note  > > looks fine if you can add a whatsnew note as @charlesdong1991 suggests and ping on green (bug fix section of 1.3 in visualization)
> 
> Added whatsnew note

did you push ? it doesn't show up > > > looks fine if you can add a whatsnew note as @charlesdong1991 suggests and ping on green (bug fix section of 1.3 in visualization)
> > 
> > 
> > Added whatsnew note
> 
> did you push ? it doesn't show up

Oops ! Missed it. Pushed it now . emm, could you please do it again, seems still some docs in whatsnew are from other PRs. @yeshsurya  > emm, could you please do it again, seems still some docs in whatsnew are from other PRs. @yeshsurya

Done ! thanks @yeshsurya it looks like there is a pre-commit error about trailing space in your whatsnew, could you fix it and rebase master? ping if all green ^^ Another cleaner pull request raised for this : https://github.com/pandas-dev/pandas/pull/41349 emm, okay, @yeshsurya , then I will close this PR to make way for the other one