Linux already has a setting to toggle between the "Chrome theme" and the GTK theme for its UI.  The former completely ignores system settings, the latter tries to respect them as much as possible.

Windows walks a midway point.  We respect things like system titlebar colors, but only if the user has enabled colored titlebars.  We respect some system text and background colors/fonts/sizes, but increasingly do our own thing (e.g. the omnibox text selection background color is still the system color, but Refresh moved away from basing the selected item background in the omnibox dropdown on that color).

In Refresh, respecting system settings comes with downsides; because background tabs are not given a unique background color, a brightly-colored system titlebar can make favicons harder to pick out.  Accordingly, some users have been asking for a way to make Chrome ignore their system settings (see e.g. https://www.reddit.com/r/chrome/comments/9e8cze/bugs_does_nobody_in_chrome_team_or_anything_test/e5nbkmi ).

We could add a theme in the webstore to match the default theme, but there are problems:
(1) Chrome themes can't actually do everything the default theme does; for example, they can't theme the omnibox.
(2) We'd have to update this theme every time we change the UI.

An alternate is to do what Linux is doing and just add a toggle for "Chrome vs. system".  Engineering-wise, this ought not to be too hard, because some of the machinery is already built; we'd just need some conditionals in the theme code about which colors to select.

UI-wise, there are some benefits.  The designers are constantly wanting more freedom from system constraints, and this could provide it, while letting people like me who value system fidelity choose that.  We could do things like use Google-specific fonts in the "Chrome" theme and stick with Segoe UI and similar in the system theme, and we could even decide to use different window metrics, close button shapes, or other more radical changes, if we thought it was a win.

However, this also requires us to test and maintain two variants, each for a smaller fraction of users.  Some questions are unclear: for things like fonts or (today) omnibox colors, which themes don't control, does any custom theme you install get layered atop the "chrome" or "system" base you chose?  Or do we always switch to one or the other as the base for a custom theme, and ignore your pref?  Which way do we default this flag?  If we add Win 10 dark mode support, would it only affect "system" mode?

->markchang as the relevant-seeming PM to decide whether this has any merit.