Right click on that login page and click view source and copy it into a notepad and save as html file. They have implemented several operations like signup, password help etc. in the single page. So you need to find only login section and delete the rest of them. Just keep the following style sheets. You can change these files as you want,
<link rel="stylesheet" type="text/css" href="https://secure.assets.tumblr.com/stylesheets/compressed/global.css?cb1e8b42147f30c2e13d62135690991a"><link rel="stylesheet" type="text/css" href="https://secure.assets.tumblr.com/stylesheets/compressed/onboarding.css?1adfaf6588f389f7567dae907f646280"><link rel="stylesheet" type="text/css" href="https://secure.assets.tumblr.com/stylesheets/compressed/dashboard.css?bccedfaa1a01c28c280ee47200935a38"><link rel="stylesheet" type="text/css" href="https://secure.assets.tumblr.com/stylesheets/compressed/hello_tags.css?17d5a86bd54c60a37ef98400e6b86b4c"><link rel="stylesheet" type="text/css" href="https://secure.assets.tumblr.com/stylesheets/compressed/notification_new.css?9003193efe4adc94a3d8c85421fa6403">
Once you just have the login section in html, you can have the login page same as Tumblr login.