Answer :
Just add this code to your text area and you are ready to customize from first moment -
<style> 
.sfc-action-button {                       
color: #ffffff !important;                 
font-size: 26px !important;                
height: 36px !important;                   
background-color: #858585!important;       
background-image: none !important;         
}                                          
</style> 
In case, just in case, If HTML Sensitization is enabled by your admin - You are bit unlucky. Still you can do a few stuffs using JavaScript codes. A Small Example code here-$(".sfc-action-button").css({
 "width":"600 px",
 "font-size": "200%"});