Opacity control on post background

I wanted to include semi-transparent background in a custom color scheme. Right now, only preset color scheme 7 has transparent background, and I like it but want different color fonts.

I created a child theme (wp/wp-content/themes/eino-child/style.css).
I have tried dropping snippets of css copied and altered from the parent theme to create an RGBa background like this:

/* CSS Document */
/*
Theme Name: eino child
Theme URI: https://roulezcycles.com/wp
Description: Eino Child Theme
Author: Jay Borden
Author URI: https://roulezcycles.com
Template: eino
Version: 1.0.0
Tags:
Text Domain: eino-child
*/

/* =Theme customization starts here
.breadcrumb-trail,
.loop-meta,
.hentry,
.comment-wrap,
#respond,
#menu-primary,
#sidebar-primary .widget,
#sidebar-secondary .widget,
#sidebar-front-page .widget-inside,
#sidebar-subsidiary,
.loop-nav a,
.pagination .page-numbers,
.page-links a,
a.more-link {
background: #000;
background: rgba(0,0,0,0.5);
}

But it seems to have no effect on the presentation of the theme.

Can you offer any help?

Thanks

jb