Hi there,
first off you have several PHP codes snippets in your child theme style.css
Every line before this line is PHP:
// STYLE FOR CUSTOM SELECTION COLOR
PHP needs to be added to your child themes function.php
More info here:
https://docs.generatepress.com/article/adding-php/
Secondly this // STYLE FOR CUSTOM SELECTION COLOR
is a valid comment in PHP but it is not valid for CSS. Instead you use:
/* This is a valid CSS Comment */
So remove the PHP and the invalid comments and try this CSS:
/* Change selection colors */
::selection {
background: #ed008c !important;
color: white;
}
/* Change Related Post widget title color */
h3.rpwe-title a {
color: #ed008c !important;
}