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Hello @mercanox,

You can learn how to use Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS:
https://developers.google.com/web/tools/chrome-devtools/
https://developer.mozilla.org/en-US/docs/Tools
CSS Tutorial: https://www.w3schools.com/css/

And for responsive sizes, put your CSS on custom screen size:

@media only screen and (max-width: 480px) {
    /* put you CSS here*/
}

Or use between sizes:

@media (max-width: 960px) and (min-width: 481px){
    /* put you CSS here */
}

For more information about media queries, please read this article:
https://www.w3schools.com/css/css_rwd_mediaqueries.asp.

Best Regards