Can you support setting custom backdrop color?

[mygu]

Can you support setting custom backdrop color?

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report 
[x] Feature request
[ ] Documentation issue or request

Current behavior

Now, the backdrop option can only support true or false

Expected behavior

I want to set the backdrop option custom background color, it's look like:

var viewer = new Viewer(element, {
  backdrop: 'rgba(255, 255, 255, 0.7)'
});

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

The current default background color is not friendly to some transparent images

Environment

Viewer.js version: X.Y.Z


Browser:
- [x] Chrome (desktop) version 68.0.3440.84
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX

Others:

[fengyuanchen]

Why not just override the existing background color in your own code?

.viewer-backdrop {
  /* Your custom background color here. */
  background-color: rgba(0, 0, 0, .5); 
}

[mygu]

Why not just override the existing background color in your own code?

.viewer-backdrop {
  /* Your custom background color here. */
  background-color: rgba(0, 0, 0, .5); 
}

I tried it, but it changes the overall background color. What I need is some pages, some of which have a black background and some white background.

[fengyuanchen]

@mygu A new option className has been added to Viewer.js (v1.3.0) for your case.

[mygu]

@fengyuanchen Thanks a lot :)