/etc/webmin/authentic-theme/styles.css

cat > /etc/webmin/authentic-theme/styles.css << 'EOT'
#sidebar{
background:#1689c0 !important;;
color: white !important;
box-shadow: none !important;
opacity: 1;
}
 
#webmin_search_form input[name]:not([type=image]):not([type=checkbox]):not([type=radio]):not(.btn):not(.session_login) {
  height: 32px;
  filter: brightness(0) invert(1);
}
aside .form-group i.fa.fa-search {
    color: white;
}
#sidebar li:not(.menu-container):not(.sub-wrapper) a {
    color: white;
}
 
#sidebar li:not(.menu-container):not(.sub-wrapper) a:hover {
    text-decoration: underline;
}
#sidebar li:not(.sub-wrapper):not(.menu-container).sub_active {
    border-left: 4px solid gold;
}
#sidebar ul.user-links li:not(.sub-wrapper):not(.menu-container)>a, #sidebar ul.user-links li:not(.sub-wrapper):not(.menu-container)>span {
    color: white;
}
#sidebar li:not(.sub-wrapper):not(.menu-container).has-sub.active {
    background-color: gold;
}
 
#sidebar li:not(.menu-container):not(.sub-wrapper).active a {
    color: #333;
}
#sidebar .select2, #sidebar .select2-container, #sidebar .select2-container--default {
    color: white;
    background-color: #333;
}
.switch-mins a {
  border-radius: 0;
  background-color: #1689c0;
}
.btn-info {
    background-color: #1689c0!important;
}
.btn-primary {
    border:none;
    background-color: #1689c0!important;
}
#sidebar li.sub-wrapper {
background-color: #fff1b5!important;
}
#sidebar li.sub-wrapper a {
color: #333 !important;
}
#sidebar li:not(.menu-title):not(.menu-container):not(.user-html-string) {
    text-shadow: none;
}
#sidebar ul.sub>li>a:before {
    color: black !important;
}
#sidebar ul.sub li:before {
    background: gold;
}
 
#sidebar ul.sub>li:not(.menu-container).sub_active:before {
    background: black;
}
.wbm-webmin:before {
    content: '\f00a';
}
.wbm-virtualmin:before {
    content: '\f03b';
}
EOT