Hi,
always the best thing is to check the affected classes by an web development tool. This is included in each web bowser.
You find this for the "Previous"-Button: class="ls-move-btn ls-move-previous-btn btn btn-lg btn-default".
The only class which only addresses this button is: "ls-move-previous-btn"
So you can use it to change the appearance of the button.
In your custom.css add:
.ls-move-previous-btn {
background-color:[your_desired_color] !Important;
color:[your_desired_textcolor] !Important;
border:[your_desired_borderstyle] !Important;
...
}
Th same for ".ls-move-next-btn"
Regards
Joffm