Control-group element borders don't necessarily align in dark theme

[cmslewis]

Bug Report

• Package version(s): v1.1.0
• Browser and OS versions: Chrome 54.0.2840.71 (64-bit) on OS X 10.11.6

Describe the issue:

In the dark theme, intent-ful control group elements don't necessarily have aligned borders. See this example from the numeric stepper:

Actual behavior:

Note that the red input-field border doesn't align with the red buttons' edges.

Expected behavior:

Would be nice if they did align. Sort of a tricky case though.

[cmslewis]

@llorca would love your thoughts here

[llorca]

Good catch! I'll look into it, numerical stepper revealed a few visual bugs around dark theme control groups