Hello,
To make the outline dark you need to add the following Custom CSS in the Global Custom CSS area under Theme Settings >> Custom CSS.
.searchform input[type="text"] {
border: 2px solid black;
}
If you want to make the background dark then you need to use the following property in the Custom CSS above.
"background: black;"
If you want to ask anything else then provide some relevant screenshots of the specific area to check it myself and help you out accordingly.
Best Regards.