Normal Button

Normally by default the buttons looks like primary WordPress buttons.

GLFW manual
._df_button {
 //your style goes here
 }

Changing Color

GLFW manual
._df_button {
     background: grey !important;
     border: none !important;
     color: white !important;
 }
 ._df_button:hover {
     background: #444 !important;
 }

Custom Class

Extra classes can be used to change style for certain buttons only.

GLFW manual
.red_button {
     background: crimson !important;
     border: none !important;
     color: white !important;
 }
 .red_button:hover {
     background: red !important;
 }