Please open the theme.php file and have a look at the following part:
addbutton($sub_menu_buttons,'{CUSTOM_LNK_LNK}','{CUSTOM_LNK_TITLE}','{CUSTOM_LNK_TGT}','custom_link',$template_sub_menu_spacer);
addbutton($sub_menu_buttons,'{ALB_LIST_LNK}','{ALB_LIST_TITLE}','{ALB_LIST_TGT}','album_list',$template_sub_menu_spacer);
addbutton($sub_menu_buttons,'{LASTUP_LNK}','{LASTUP_TITLE}','{LASTUP_TGT}','lastup',$template_sub_menu_spacer,'rel="nofollow"');
addbutton($sub_menu_buttons,'{LASTCOM_LNK}','{LASTCOM_TITLE}','{LASTCOM_TGT}','lastcom',$template_sub_menu_spacer,'rel="nofollow"');
addbutton($sub_menu_buttons,'{TOPN_LNK}','{TOPN_TITLE}','{TOPN_TGT}','topn',$template_sub_menu_spacer,'rel="nofollow"');
addbutton($sub_menu_buttons,'{TOPRATED_LNK}','{TOPRATED_TITLE}','{TOPRATED_TGT}','toprated',$template_sub_menu_spacer,'rel="nofollow"');
addbutton($sub_menu_buttons,'{FAV_LNK}','{FAV_TITLE}','{FAV_TGT}','favpics',$template_sub_menu_spacer,'rel="nofollow"');
if ($CONFIG['browse_by_date'] != 0) {
addbutton($sub_menu_buttons, '{BROWSEBYDATE_LNK}', '{BROWSEBYDATE_TITLE}', '{BROWSEBYDATE_TGT}', 'browse_by_date', $template_sub_menu_spacer, 'rel="nofollow" class="greybox"');
}
addbutton($sub_menu_buttons,'{SEARCH_LNK}','{SEARCH_TITLE}','{SEARCH_TGT}','search','');
As you can see, some of the buttons have a 7th parameter
,'rel="nofollow"'
That's the place where you can add your additional css classes. E.g. change the mentioned code to
addbutton($sub_menu_buttons,'{CUSTOM_LNK_LNK}','{CUSTOM_LNK_TITLE}','{CUSTOM_LNK_TGT}','custom_link',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_custom_link"');
addbutton($sub_menu_buttons,'{ALB_LIST_LNK}','{ALB_LIST_TITLE}','{ALB_LIST_TGT}','album_list',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_album_list"');
addbutton($sub_menu_buttons,'{LASTUP_LNK}','{LASTUP_TITLE}','{LASTUP_TGT}','lastup',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_lastup"');
addbutton($sub_menu_buttons,'{LASTCOM_LNK}','{LASTCOM_TITLE}','{LASTCOM_TGT}','lastcom',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_lastcom"');
addbutton($sub_menu_buttons,'{TOPN_LNK}','{TOPN_TITLE}','{TOPN_TGT}','topn',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_topn"');
addbutton($sub_menu_buttons,'{TOPRATED_LNK}','{TOPRATED_TITLE}','{TOPRATED_TGT}','toprated',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_toprated"');
addbutton($sub_menu_buttons,'{FAV_LNK}','{FAV_TITLE}','{FAV_TGT}','favpics',$template_sub_menu_spacer,'rel="nofollow" class="my_theme_btn_favpics"');
if ($CONFIG['browse_by_date'] != 0) {
addbutton($sub_menu_buttons, '{BROWSEBYDATE_LNK}', '{BROWSEBYDATE_TITLE}', '{BROWSEBYDATE_TGT}', 'browse_by_date', $template_sub_menu_spacer, 'rel="nofollow" class="greybox"');
}
addbutton($sub_menu_buttons,'{SEARCH_LNK}','{SEARCH_TITLE}','{SEARCH_TGT}','search','','rel="nofollow" class="my_theme_btn_search"');
and use that classes in your css file.