@ADN: You cannot change that just from Custom CSS. For that you need to change the code. So, for this you need to build child theme http://catchthemes.com/blog/create-child-theme-wordpress/. Then copy catchkathmandu_content_nav() function from template-tags.php to your child theme functions.php file.
Then you will see the following code:
<?php previous_post_link( '<div class="nav-previous">%link</div>', '<span class="meta-nav">' . _x( '←', 'Previous post link', 'catchkathmandu' ) . '</span> %title' ); ?>
<?php next_post_link( '<div class="nav-next">%link</div>', '%title <span class="meta-nav">' . _x( '→', 'Next post link', 'catchkathmandu' ) . '</span>' ); ?>
You need to replace with following:
<?php previous_post_link( '%link', '<div class="nav-previous">Previous</div>' ); ?>
<?php next_post_link( '%link', '<div class="nav-next">Next</div>' ); ?>
.
