Good luck finding someone who will implement full GTK theming in Chromium. I mean that sincerely. Here's why:
In ye olden days (i.e., about 12 years ago), Chromium actually fit in with the current GTK theme, sort of like it does on Windows even today. For some reason however, somebody decided GTK theming looked bad and removed the support. Now Chromium has very little reliance on GTK and isn't themed like GTK. In a way, Firefox has slowly been stepping in that direction for a long time now. One of the few browsers that does still have good GTK theming is Pale Moon, which was forked off from Firefox many years ago and has something of a... let's just say strong following by some and strong disrespect by others.
(Little off-topic note here: You can use Pale Moon if you want to, but since the project is run by only a handful of developers, you really cannot expect the browser to work on many modern Web pages. Furthermore, if you join the forum that they have and start posting there, be very careful about what you say. Many people, myself included, have posted there and made a few little mistakes, and now our reputations are soiled over a few lousy questions; there are some very, very abrasive individuals on the forum who use all kinds of deceptive, back-handed tricks to soil your reputation as far as the forum goes. However, the developers strictly insist that I say to you: I can't influence your decision making, so make up your own mind.)
All right, that was a little off topic. The most common reason for dropping GTK theming support that I've seen cited is that: GTK-based desktops used to be used by the majority of users 12 years ago, but no longer. KDE (which is based around a rival toolkit named Qt) is, according to some estimates, far more popular than GTK-based desktops today. As such, and considering that operating systems like Windows have their own officially solicited widget toolkits (whereas Linux does not), in some ways it's no wonder that browser designers stopped theming their applications using GTK; Qt is just as, if not more popular.
That's just my two cents, for whatever it's worth. Sure, you can probably convince somebody to write the necessary GTK support for you. But you can't expect your average Joe / Jane Blow Chromium developer to write it without a very convincing argument.