Hello,
This actually would involve some custom changes which isn't generally something we provide support for. I will explain the files below that you can change. The highlight_1 color is echoed to multiple elements in the index.php with the following block:
.button:hover, .s5_button:hover, .s5_highlight2, a.readon:hover, p.readmore a:hover, .s5_is_css_wrap_7 .s5_is_css_hidden a.darkcolor:hover, li.pagenav-prev a:hover, li.pagenav-next a:hover, .s5_ls_search_word, #s5_ls_outer_wrap .s5_ls_readmore:hover, #s5_accordion_menu h3:hover, .validate:hover, .pages_highlight_box, .module_round_box-highlight1, #s5_responsive_mobile_drop_down_wrap button:hover, #s5_responsive_mobile_drop_down_wrap .button:hover, .pager a:hover, .btn:hover,
.chzn-container .chzn-results li:hover, .icon_hover, .social_icon:hover, .ion-play-size:hover, .circle_table_icons:hover, .ac-container input:checked + label, .ac-container input:checked + label:hover, .ac-container2 input:checked + label, .ac-container2 input:checked + label:hover, .pager a:hover, .btn:hover, .chzn-container .chzn-results li:hover, .module_round_box-highlight_border:hover, .module_round_box-highlight_noborder:hover, .horzaccordion .image_title a.btn, .s5_pricetable_column.recommended, #myGallery a.readon, #searchForm button.btn, div.catItemCommentsLink a:hover, div.catItemReadMore a:hover {
background-color: #fff000 !important;
}
The background color used above is echoed from the Template Manager highlight color choice but also has a !important tag so it cannot be overridden. You would need to go into the index.php file and find the code block above and change the PHP echo to something with an opacity. So instead of the #fff000 it would be something like so:
background-color: rgba(54, 25, 25, 0.5);
Where 0.5 is the opacity level.
Regards,