The ability for omnibox colour to be inferred from the titlebar colour was removed in Chrome v81. As a result, Chrome themes need to explicitly specify omnibox colour to continue looking the same. Theme version 5.2.0 contains this update.
With this release, I have also moved this theme to "deprecated" status. It's been fun updating this theme, but this theme has long been superseded by the official Google Chrome dark mode. I recommend enabling the dark mode and uninstalling this theme - this will give you the best experience.