Repository: matplotlib/matplotlib
Pull request: Changed line color of boxplot for dark_background
## PR Summary

Rework of #15705. Closes #15675. 
Previously, the black outlines of boxes in boxplots were hidden inside the black background in the `dark_background` style.

Now: Outputs of boxplot_demo.py (https://matplotlib.org/examples/pylab_examples/boxplot_demo.html) using dark_background are as shown below:
![image](https://user-images.githubusercontent.com/43081866/69155425-b22a1380-0b07-11ea-80ff-b7af8d222b2c.png)
![image](https://user-images.githubusercontent.com/43081866/69155436-b7875e00-0b07-11ea-810d-9a574d45e5c9.png)
![image](https://user-images.githubusercontent.com/43081866/69155452-c241f300-0b07-11ea-84e9-50daa42176af.png)
![image](https://user-images.githubusercontent.com/43081866/69155461-c66e1080-0b07-11ea-9dd4-1ce7e0fd03d2.png)
![image](https://user-images.githubusercontent.com/43081866/69155466-c8d06a80-0b07-11ea-871d-2879cc7ed3cc.png)
![image](https://user-images.githubusercontent.com/43081866/69155472-cb32c480-0b07-11ea-80b5-febbbc72edf1.png)
![image](https://user-images.githubusercontent.com/43081866/69155482-cd951e80-0b07-11ea-9fa7-b2de08c2955f.png)


## PR Checklist

- [ ] Has Pytest style unit tests
- [ ] Code is [Flake 8](http://flake8.pycqa.org/en/latest/) compliant
- [ ] New features are documented, with examples if plot related
- [ ] Documentation is sphinx and numpydoc compliant
- [ ] Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
- [ ] Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

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Problem statement:
Boxplot line color with style dark_background should be bright
<!--To help us understand and resolve your issue, please fill out the form to the best of your ability.-->
<!--You can feel free to delete the sections that do not apply.-->

### Bug report

```
%pylab inline
plt.style.use('dark_background')
plt.boxplot([1,2,3])
```

The line color should not be black. With the dark_background style the boxes are not visible.

Hints from issue discussion:
### **Normal:**
**Code:**
`%pylab inline`
`plt.boxplot([1,2,3])`
**Result:**
![image](https://user-images.githubusercontent.com/43081866/68996832-799af780-08c5-11ea-9277-6a3184d96f90.png)

### **Dark Background:**
**Code:**
`%pylab inline`
`plt.style.use('dark_background')`
`plt.boxplot([1,2,3])`
**Result:**
![image](https://user-images.githubusercontent.com/43081866/68996870-e31b0600-08c5-11ea-9e36-1b45b04104d2.png)

I tried to figure out how can we solve this problem. But unfortunately, I wasn't successful. It would be great if someone can guide me.
I think that the `dark_background` style (https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle) also needs to set the boxplot rcparams


```
#### Boxplot
#boxplot.notch       : False
#boxplot.vertical    : True
#boxplot.whiskers    : 1.5
#boxplot.bootstrap   : None
#boxplot.patchartist : False
#boxplot.showmeans   : False
#boxplot.showcaps    : True
#boxplot.showbox     : True
#boxplot.showfliers  : True
#boxplot.meanline    : False

#boxplot.flierprops.color           : black
#boxplot.flierprops.marker          : o
#boxplot.flierprops.markerfacecolor : none
#boxplot.flierprops.markeredgecolor : black
#boxplot.flierprops.markeredgewidth : 1.0
#boxplot.flierprops.markersize      : 6
#boxplot.flierprops.linestyle       : none
#boxplot.flierprops.linewidth       : 1.0

#boxplot.boxprops.color     : black
#boxplot.boxprops.linewidth : 1.0
#boxplot.boxprops.linestyle : -

#boxplot.whiskerprops.color     : black
#boxplot.whiskerprops.linewidth : 1.0
#boxplot.whiskerprops.linestyle : -

#boxplot.capprops.color     : black
#boxplot.capprops.linewidth : 1.0
#boxplot.capprops.linestyle : -

#boxplot.medianprops.color     : C1
#boxplot.medianprops.linewidth : 1.0
#boxplot.medianprops.linestyle : -

#boxplot.meanprops.color           : C2
#boxplot.meanprops.marker          : ^
#boxplot.meanprops.markerfacecolor : C2
#boxplot.meanprops.markeredgecolor : C2
#boxplot.meanprops.markersize      :  6
#boxplot.meanprops.linestyle       : --
#boxplot.meanprops.linewidth       : 1.0


```

Commit message: ### Boxplots boxplot.boxprops.color: w boxplot.capprops.color: w boxplot.flierprops.color: w boxplot.flierprops.markeredgecolor: w boxplot.meanprops.color: g boxplot.medianprops.color: g boxplot.meanprops.markerfacecolor: g boxplot.meanprops.markeredgecolor: w boxplot.whiskerprops.color: w

Patch to review:
diff --git a/lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle b/lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle
index a8b754005bfd..7ee59535f9c1 100644
--- a/lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle
+++ b/lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle
@@ -21,3 +21,13 @@ figure.edgecolor: black
 savefig.facecolor: black
 savefig.edgecolor: black
 
+### Boxplots
+boxplot.boxprops.color: w
+boxplot.capprops.color: w
+boxplot.flierprops.color: w
+boxplot.flierprops.markeredgecolor: w
+boxplot.meanprops.color: g
+boxplot.medianprops.color: g
+boxplot.meanprops.markerfacecolor: g
+boxplot.meanprops.markeredgecolor: w
+boxplot.whiskerprops.color: w


Code review comments:
diff_hunk: @@ -21,3 +21,13 @@ figure.edgecolor: black
 savefig.facecolor: black
 savefig.edgecolor: black
 
+### Boxplots
+boxplot.boxprops.color: w
+boxplot.capprops.color: w
+boxplot.flierprops.color: w
+boxplot.flierprops.markeredgecolor: w
+boxplot.meanprops.color: g
path: lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle
text: @user1:
I would prefer to use a color from the current color cycle. That makes a more homogeneous appearance than hard-coding a color.

diff_hunk: @@ -21,3 +21,13 @@ figure.edgecolor: black
 savefig.facecolor: black
 savefig.edgecolor: black
 
+### Boxplots
+boxplot.boxprops.color: w
path: lib/matplotlib/mpl-data/stylelib/dark_background.mplstyle
text: @user1:
Please use `white`. While technically `w` is allowed, we try to keep style files more readable.