How do I add extra files (in my case, .css file) to a userscript in here?

Only user.js and .js files are currently hosted... anything else you may use @require and/or @resource in your metadata block pointing to a data URI or elsewhere on the web, or embed into your source code. Please note there is currently a maximum size of approximately 500KiB script size... so best to use the metadata block to point elsewhere.

I use the Marti solution with the help of its GM_setStyle.js for this script:
Userstyles TableView+Enhancer - Dark/Grey

I post my CSS to Pastebin (by example for this script:
and add into my userscript header:

// @require
// @resource 
// @grant         GM_getResourceText

after i use this code:

let styleNode = GM_setStyle({
    data: GM_getResourceText("css")