erd? none exists. want to know structure? read 'all' code & knowing will come.. in time
framework? elgg is the framework.

tools exist to auto generate diagrams from mysql database scripts.. 

@Dhrup and @ura thanks for your responses they really helped. thanks

There is not a good way to do this now. You can vote this ticket to be fixed: http://trac.elgg.org/ticket/1919, but pull requests are going to make things happen the fastest.

What is line 50 of ../views/default/core/walled_garden/login.php? Obviously something is wrong in this very line of code so you should check it out or post only this line so we can debug it.

thanks. i have figured it out. thanks.

CSS is generated from views/default/css/*. Use the developer tools plugin bundled with Elgg and turn on view comment wrapping to find out what views are generating which part of the page.

docs.elgg.org is your friend. Let us know if any info is missing from there.

Thanks Evan.