The place does not really matter, as long as it is in your theme folder. I would suggest to create a separate folder in your theme folder, like: "YourProject/theme/styles/mxui.css”. Once the css file is available in your theme folder you can simply import it in your Sass files or include it in your HTML snippets/pages. Do keep in mind that Mendix 8 uses a totally different UI approach, and that importing the mxui.css could therefore create some conflicts or unwanted styling.
Hope this helps,