I usually have three chrome windows open. To easily distinguish my work chrome from the personal chrome window, I would like to set a theme to one window, and another theme to other windows.
I figure out how to change the theme globally, wondering whether its possible to it per window.
Use different profiles. You can do this in chrome by clicking the Proflile icon in the top-right of any window. It'll ask you to log in with your Google account but you're able to make a local account with any name.
I use one for personal (personal google account), one for work (work google account), and a third with no google account for risky or other website browsing (which has more aggressive blocking rules)