The CSS you need appears to have been commented out!
Try removing the /* and */ as highlighted below, and remove that small section of CSS you added as it is included in this commented out list as you will see...
/*
.editing .section:hover .section_add_menus,
.editing .block:hover .section_add_menus,
.editing .sitetopic:hover .section_add_menus,
.editing .block:hover .addresourcemodchooser,
.editing .section:hover .addresourcemodchooser,
.editing .sitetopic:hover .addresourcemodchooser,
.editing .block:hover .title .commands .icon,
.editing .block .title .commands .icon:active,
.editing .block .title .commands .icon:focus,
.editing .block .content li:hover .commands a,
.editing .block .content li .commands a:active,
.editing .block .content li .commands a:focus,
.editing .block:hover .editbutton a,
.editing .block .editbutton a:active,
.editing .block .editbutton a:focus,
.editing .section:hover .side > *:not(br),
.editing .section .side > *:active,
.editing .section .side > *:focus,
.editing .section .summary:hover a:first-child,
.editing .section .summary a:first-child:active,
.editing .section .summary a:first-child:focus,
.editing .section .sectionname:hover + .summary a,
.editing .section .activity:hover .commands > *,
.editing .section .activity .commands > *:active,
.editing .section .activity .commands > *:focus,
.editing .sitetopic:hover > a,
.editing .sitetopic > a:active,
.editing .sitetopic > a:focus {
filter: none;
opacity: 1;
}
*/