Yeah. Use Firebug to look at the button you want to replace, and then replace the code in custom CSS.
In other words, you have code like this already:
.my_button {style elements that I came up with;}
.contact_7_button {boring looking style elements;}`
and you would add this to your custom css or child theme stylesheet:
.contact_7_button {style elements that I came up with;}
Firebug will help you figure out what the actual class or id name the Contact 7 plugin (presumably?) is using. I can guarantee it isn’t .contact_7_button
.