removed color treatment of outline for easier integration

[masterbee]

• The default outline treatment of outline is invert which ensures that the outline colour will always have sufficient contrast from the background it is on.
• .tab-focus() mixin was styling the background colour to #333 , this poses a bit of a restriction for implementers of twitter bootstrap since if the background is darker than white, there may not be enough contrast to see the outline, causing an accessibility issue.

Great work again TB team, looking forward to v3.0.3

/cc @mdo @cvrebert

[cvrebert]

I have no opinion personally, but I am going to point out that I've seen complaints from some perfectionists who find the default dotted outlines ugly; the special WebKit styling throws them a bone in that regard.

[masterbee]

@cvrebert - Thanks for chime in, and I am all for throwing a bone when we can, but since outline is one of those areas branded by accessibility [http://www.w3.org/TR/WCAG-TECHS/F78.html], styling colour in that property maybe a slippery slope, when natively browsers are already doing it for you safely, IMHO.

Keep up the great work guys 👍

[mdo]

I don't think I want to remove the WebKit outline, but I can get behind removing the color from the default outline.

[masterbee]

@mdo , thanks for the response. will update my PR to address.

[masterbee]

@mdo , @cvrebert I will be tweaking this in a few moments. Sorry about the delay

Thank you again.

[masterbee]

@cvrebert , @mdo - PR updated.

[cvrebert]

@masterbee Could you squash this into a single commit? If not, I can, when @mdo gives the go-ahead.

[masterbee]

@cvrebert - all set thanks for the patience.