New gutter blends into collapse indicator

[tolmasky]

I'm not sure if this was the intent, but without the gutter borders, the gutter can end up "blending into" the collapse indicator, leading it to feel like a weird geometric shape:

It's actually ever so slightly a different color (230,230,230 vs. 229,229,229), but I can barely tell. If it's important to keep the gutter borderless, then maybe we can at least make sure the collapse indicators always have a sufficiently different color? As is though, those areas don't even appear clickable to me, they just seem to be parts of the gutter.