what exactly are trying to edit and customize?

JIt seems like Actitivity is a static page so I want to make my own home/members page, but I cant cause Activity is there.. I  want to completely customize it.. or  somehow  replace the tabs page under "Activity" right where it says "All" "Mine" "Friends" ...    To completely customize the Acitivity Page Or To make another tab page under Activity will do... 

the activity page is made of of several different views if i remember correctly.  the tabs are a filter, then there is the river, then the sidebar or bars depending if you are using a 2 or 3 column layout.  you might want to look into a couple of plugins such as river_addon or river_activity_3C.  several very good plugins that will customize the layout of the activity page or all pages.  i personally use river activity 3C with a updated customstyle plugin that allows me to control they layout and my users to customize  the colors and backgrounds.  will send you a pm with address, username and password if you want to see it

the best away to customize the activity page with my theme is editing the views / default / page / default.php file and inserted this code in the appropriate line

if((elgg_get_context() == 'activity')) {
echo "<div clas=\"elgg-main\">";
echo "<div clas=\"elgg-inner\">";
echo elgg_view('demyx/activity');
echo "</div>";
echo "</div>";
}

that code calls for the activity.php file inside mod/demyx/views/default/demyx/activity.php and i basically put whatever i want in there