Highlights Problems

Add the following code to the custom CSS of your child theme:

::selection {
	background-color: #333!important;
	color: #fff !important;
}

Of course, you can change it to any color code you like.
I recommend you to use a color palette.

Still nothing changed, edited it on the same place as ol ::selection was, in style.css .
In addition, how to solve this hyperlink and author issue with, the same, #fff white text color.

Well, it’s about the only way to do it so there must be something overriding it.
I can see that you’re not using a child theme. You can download it here and change the stylesheet of your childtheme.

If you want to change the color of your website’s hyperlinks, you can use the following code to do that:

a {
    color: #00ff00 !important;
}

Like with highlighting, you can change the color to anything you like.