CommerceGurus

First add this plugin:

https://wordpress.org/plugins/simple-custom-css/

Then, within Appearance > Custom Code add the following:

.mobile-search-reveal input.search-field,
.mobile-search-reveal input.search-field:focus {
  color: #333;
}

.mobile-search-reveal input.search-field::-webkit-input-placeholder {
    color: #333;
}

.mobile-search-reveal input::-moz-placeholder {
    color: #333;
}

.mobile-search-reveal input:-ms-input-placeholder {
    color: #333;
}