BUG : Min/max markers on box plot are not visible with 'dark_background' (#40769)
- [x] closes #40769
- [x] tests added / passed
- [x] Ensure all linting tests pass, see here 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/9417467/117279259-2634b480-ae7f-11eb-933c-350cff0133de.png)](https://user-images.githubusercontent.com/9417467/117279207-1a48f280-ae7f-11eb-9f0f-fe477d80dc77.png)


After Fix :

Caps for the box plow will be visible in dark background mode :
![image](https://user-images.githubusercontent.com/9417467/117279282-2df45900-ae7f-11eb-8107-9fe3ae2fe15f.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.
sorry, @yeshsurya a bit unlucky, I _think_ this failure isn't related to your change, probably you will need another rebase  thanks very much, very nice!