It is possible to apply custom CSS rules to the order lookup Form.

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

Simply use the following 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.

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