So for enterprise development you need a ponyfill to support this.
Otherwise, you could certainly set up your theme.css to have a set of CSS Variables you reuse in the rest of your CSS files, then dynamically change between light-theme or dark-theme or other custom themes with JS
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
CSS Variables for one aren't supported in IE11 : caniuse.com/#feat=css-variables
So for enterprise development you need a ponyfill to support this.
Otherwise, you could certainly set up your theme.css to have a set of CSS Variables you reuse in the rest of your CSS files, then dynamically change between light-theme or dark-theme or other custom themes with JS