Discard bordered URL bar in favour of borderless URL bar

In Safari, the URL bar is borderless as you can see here:

Compared to Orion's URL bar, which is bordered, and less pretty by comparison:

The borderless effect is achieved by using a slightly darker variant of the toolbar's accent color for the background of the URL bar (but you already knew that)

It would just make Orion look better. This, along with my previous post on splitting the URL bar from the profile switcher ( #9977 ) would serve to make Orion look much better.