ENH: Add light background to inline code

[choldgraf]

This adds a lightweight background to our inline code to follow a pattern that is very common in other documentation platforms, and hopefluly makes inline code standout better.

Two quick points:

• It doesn't change the code color at all, do people think we should change the color to be the same as our text?
• It tries to add a light border that isn't intrusive, @drammock is it OK to you?

Here's an example in light/dark:

closes #837

[drammock]

Yes, border is acceptably subtle :). Thanks! Re: color, my vote is that it's better to have inline code a different color from both normal text and from links

[12rambau]

as mentioned in the related issue, I like the pinky inline code of this theme.

LGTM

[choldgraf]

OK cool - since we've got two approvals and there's general preference for leaving the color as-is, let's do that and people can change the color on their own if they wish.