Button focus outline color

[ysds]

I prefer the button's focus outline color is different from the background(border) color. Because the outline can not be seen in the case of a solid button group (btn btn-primary etc.).

Current:

Lighter example:

[andresgalante]

Focus colors are always variables. For example for btns:
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L311-L312

You can always change them and recompile if you want.

[ysds]

Yes. But I think it should be visible by default.

[mdo]

Unsure how best to proceed on this... perhaps we saturate or tint the focus shadow instead of using a regualr alpha-transparent theme color?

[ysds]

@mdo Please check the PR. But it not match to outline's focus color. It is better to have the same color for consistency.