Changing Background Color Breaks Submit Button

I used the default appearance for the form and then added

#this_form .rm_button { background-color: rgb(40,40,90);}

to change the color of the submit button. At this point the button stopped working, I wasn’t able to diagnose why and simply reverted to the standard color. Not the end of the world, but I figured someone might have some advice on how to fix it.

Using latest WP and plugins.

Thanks for any help.