categories images align center and set transparent background

Hello,

The first idea, is to check that your css command is well built and with a good priority, I detail my idea :
1. First the minimum necessary to work properly :
Check that your selector is well written, good class (or Id) html and no mistake or typo
If your command is visible in your browser inspector tool, but is overridden (css command crossed out), then his priority isn't strong enough.

.your_required_container_class {
     background-color: rgba(0,0,0,0.5);
}

Hello,

First, your img is in block a quick solution is to display it in "inline-block"

.hikashop_category_image img {
    display: inline-block;
}

.hikashop_subcategories_listing .hikashop_subcontainer.thumbnail {
    background-color: rgba(32, 168, 198, 0.5);
}