sass-vars-loader | Use Sass variables defined in Webpack config | Style Language library

I suggest you re-think your strategy of using build tools, but I'm not convinced you need to build a server to achieve the result you want.

Maybe you can compile all of the themes, and 'swap' them out at run-time. Replacing css file on the fly (and apply the new style to the page)

If the themes are not fixed, i.e. you can have any combination, the maybe you can embed the 'theme' css in the main html page and swap out those values as required. Change CSS rule at runtime

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I fully understand that it's intimidating to have to do a major design change at this point in your applications lifecycle but I am very certain that your current approach is going to cause you signifigantly more headache.

Angular, SCSS, node etc were not intended for and do not lend themselves to the scenario which you are describing.

Are you aware that in the scenario you're describing; if any user triggers a 'theme change', the website will be 'taken offline' while it's recompiling and that the resulting css theme applied would be visible to everyone using the site, not just the user who triggered it?

Also, the browser isn't using your scss files, it's using the compiled output from all your SCSS files which I believe in ng4's case, is a JS file with the CSS embedded.

In order to do what you're proposing wihtout recompiling angular, (which will require a lot of additional back-end server work and potentially lead to more complexity/reliability issues) you will need to get and populate your vars.SCSS file, combine it with all the other SCSS files, somehow replicate how angular CLI generates the CSS embedded JS file and then load that into your browser, replacing the initial one... and you would need to do all of that from javascript/typescript running in the browser.

Redesign your app a little, create a css 'overlay' file, and at runtime, swap out the values and load it into the DOM to change themes. I'm sure others will weigh in here too, but I believe that's a much better direction to proceed to get the results you want, as quickly as possible.