Please Add CSS Variables of your UI

Please add css variables so you can edit the UI more easily. For example:

.df-ui-btn.df-active, .df-ui-btn:hover {
	color: #00acce;
	background-color: #eee;
}

Please change to:

._df_book {
--ui-color: black;
--ui-background-color: white;
}

.df-ui-btn.df-active, .df-ui-btn:hover {
	color: var(--ui-color);
	background-color: var(--ui-background-color);
}