It is possible to apply custom css rules to the order look up Form.

Here are the instructions.
1. Go to Settings -> Display Settings
2. Enter Custom CSS Rules

Simply use the follwing CSS Rule to change the color of LookUp Button.

#order-info .main-btn
{
background: navy;
font-size: 15px;
}

This rule will change the color to Navy Blue. Here is a Screenshot for your reference.

1
Similarly you can change in font , table color, font size etc using custom CSS Rules.