Hi Sebastian,
I understand your first sentence. However, for this,

ES login cover me front slide show.

What do you mean? Perhaps some screenshots would be great to further explain the details.

To override any css in EasySocial, you would have to create the custom.css file and directory here: JoomlaFolder\templates\yourCurrentTemplate\html\com_easysocial\css\custom.css
Then apply any new css rules that you want to override for EasySocial in the file.

Here is the guestpage. First page with loginbox is problem.

Hi Sebastian,
Perhaps you can try using this css code

.es-login {

    background-color: rgba(255,255,255,.4);

}

To override css rules in EasySocial, you can refer to my instructions in my previous reply.