Hi, this is very simple.
Go to “Plugins” and klick on “Plugin-Editor”. On the next, choose “EasyAppointment” on the right upper corner.
Then select on the right side “Css” and the File “eafront-bootstrap.css”. Then go to Line 255 where you can find the following code:

.ea-bootstrap .ui-datepicker .no-slots, .ea-bootstrap .ui-datepicker .no-slots:hover {
	background-color:rgba(163,163,117,0.5);
	color: #fff;
	cursor: default;
}

Now you can change “background-color:” to following Value for Dark-Grey: 

• Hex: #A9A9A9
• RGB: rgb(169,169,169,0.5) 

On RGB you could change the transparency with the last named Number (0.5). Max Value is 1.0 for “no-transparency”.
After you changed this, reload your Page with “Strg + F5”. 
Have fun with the new color 🙂

Michael wrote good solution for that one 🙂

Best regards,
Nikola