The top bar of the page, containing the product information, search, and

actions, has historically been a loose collection of CSS classes that

were a bit fragile to deal with. In the work toward dark mode, a lot was

needed to clean this up.
This is now a .rb-c-topbar component, utilizing CSS Grid for some of

the layout and flexbox for the rest. This gives us better control over

both the desktop and mobile mode layouts.
The styling now utilizes Ink CSS variables and icons, styling off the

header color. This gives us consistent, page-wide control over the

colors used in the bar.
In light mode, the colors remain the same as before. In dark mode, we

use a very dark grey, almost black, though pages can override this. The

colors here are subject to change, and is not the primary focus of this

commit.
While most of the appearance remains unchanged, one change is that the

menu item hover colors now use the accent color. This provides a degree

of consistency and avoids having to deal with too many color constants.

This overall looks quite good, and will be a standard moving forward.
The search bar also now shows an icon in all cases, which helps keep

that visually identifable.

Tested this on Chrome and Firefox in desktop and mobile modes.
Tested that the mobile menu, actions, and search functionality all

worked.
Tested in light mode and dark mode (with theme customization).