Hi,

To do that on the sidebar, please add more this code to Customize > Custom CSS:

\
.widget input[type="submit"], .widget button[type="submit"]{ color: #fff; background: #ff0000; border-color: #ff0000; }\
.widget input[type="submit"]:hover, .widget button[type="submit"]:hover{ color: #ffffff; background: #ffff00; border-color: #ffff00; }\

Change color codes #ffffff on the above code for change text color of buttons.

Change color codes #ff0000 on the above code for change the background for buttons when not hover

Change color codes #ffff00 on the above code for change the background for buttons when hover

You can get a color code here

Best Regards,

PenciDesign