How to get back my previous omnibox in Google Chrome 69 or Chromium 69?

See update at the bottom of this answer

Edit: the workaround described below was limited to version 69 and 70. It is not possible to circumvent the white background of the address bar in version 71.

Further edit: the relevant bug has now been taken up by a Googler who worked extensively on the transition of Chrome to gtk3 and so I'm hopeful of a solution soon.

Enter chrome://flags/#top-chrome-md in the omnibox and press Enter.

Click on the dropdown in the first entry, choose "Normal" and restart the browser.

It will change appearance from default

to normal

Note that flags may disappear at any time in the future. However, a bug, GTK+ theme ignored in omnibar in the new interface has been filed and, if addressed, may provide a solution without resorting to chrome://flags.

Update: This bug has not attracted sufficient interest and currently, 20181205, nobody is assigned to it. And the workaround fails in Chrome 71.

Update 20190315:

With Chrome 73, one can use --enable-features=WebUIDarkMode --force-dark-mode after making sure that the Classic theme option is chosen in Settings > Appearance instead of GTK+. Launched in this way, Google Chrome uses a built-in dark mode that doesn't rely on or use your GTK theme colors. As of now, dropdowns such as that of bookmark folders or the ☰ icon also reflect the dark mode.