By selecting theme now i get theme based html pages for login, after login, menu etc etc which is default for the theme... Actually i want my own design to incorporate. Like login page having features like create account, enabling and disabling some features from admin login is very much possible i know but what i need is not only customization but also design wise how i can implement. Please help me out. Thanks
For example i have attached one login page which has unique UI for login page.
Unless you want to modify a theme directly (not good practice), you'll need to make a child theme and override the relevant templates and SCSS. (eg. login.mustache and classes/output/core_renderer.php if you wanted to add functionality)
The theme development section here should help.