Add default background-color to SideNav

[simurai]

This adds a default background-color to the .SideNav component.

Reasoning

The main reason is to make the linter pass. 😇 Currently in our documentation we have:

<nav class="SideNav bg-gray-light border">
  <a class="SideNav-item">
  <a class="SideNav-item">

But that causes an The following CSS classes were used in class attributes but have no style rules referencing them error when used on .com. We could remove SideNav from the markup, but that feels very strange, not having a "main" class name and only use child class names.

So we need "some" property that we can use (.SideNav { ??? }). Adding background-color seems to make the most sense since we typically use a light gray background for Side Navs.

[vercel]

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[shawnbot]

This makes sense to me!