How to remove theme author's link

Theme editor is still there, only available to SuperAdmin, though.

/wp-admin/network/theme-editor.php

Dashboard->Network Admin->Themes->Editor

Creating and activating a Child Theme based on your Parent theme may be useful if you have tinkerin’ to do.

Hiding a few divs here and there can be done with Custom CSS Plugin as well.

#div_2b_hid {
display: none !important;
}

WP3.5.1 Network enabled, and you are logged in as SuperAdmin AND go to “/wp-admin/network/theme-editor.php”?

Look at your installed themes list table and click the “edit” link there, too.

wp-admin/network/themes.php

“an error”?

Lol.

Then you are not logged in as SuperAdmin or something else is limiting your role while logged in.

You need edit_themes capability which in Mulisite Network is only with the SuperAdmin.

Try a read here.

http://wordpress.org/support/topic/you-do-not-have-sufficient-permissions-to-access-this-page-125?replies=9

Does the plugin add constant to wp-congig.php?

define( 'DISALLOW_FILE_EDIT', true );

If so, then turn the security plugin setting back on/off so you can then edit files once again.