Adding class to Appointments+ buttons

Hi,

I'm developing a custom theme and I want to add some button classes to the booking page. However, I'm not sure how I would override the templates. As far as I can understand the formatting of the button is set in a class. Pointers in the right direction would be appreciated!

This is the file I'm trying to modify and among other things, adding classes to the pagination buttons. I'm using jquery right now, but I'd rather use php.

includes/class_app_shortcodes.php

// Pagination
		$c .= '<div class="appointments-pagination">';
		if ( !$month ) {
			$prev = $time - ($step*7*86400);
			$next = $time + ($step*7*86400);
			$prev_min = $appointments->local_time - $step*7*86400;
			$next_max = $appointments->local_time + ($appointments->get_app_limit() + 7*$step ) *86400;
			$month_week_next = __('Next Week', 'appointments');
			$month_week_previous = __('Previous Week', 'appointments');
		}
		else {
			$prev = $appointments->first_of_month( $time, -1 * $step );
			$next = $appointments->first_of_month( $time, $step );
			$prev_min = $appointments->first_of_month( $appointments->local_time, -1 * $step );
			$next_max = $appointments->first_of_month( $appointments->local_time, $step ) + $appointments->get_app_limit() * 86400;
			$month_week_next = __('Next Month', 'appointments');
			$month_week_previous = __('Previous Month', 'appointments');
		}

		$hash = !empty($anchors) && (int)$anchors
			? '#app_schedule'
			: ''
		;

		if ( $prev > $prev_min ) {
// Adding class here
			$c .= '<div class="previous">';
			$c .= '<a href="'. add_query_arg( "wcalendar", $prev ) . $hash . '">« '. $month_week_previous . '</a>';
			$c .= '</div>';
		}
		if ( $next < $next_max ) {
// Adding class here as well
			$c .= '<div class="next">';
			$c .= '<a href="'. add_query_arg( "wcalendar", $next ) . $hash . '">'. $month_week_next . ' »</a>';
			$c .= '</div>';
		}