You can change the theme used by following these steps:
Log in to BitBucket
Navigate to a Repository
Click the Admin tab
In the top right hand pane select Appearance Settings
Choose your highlight style from the drop down (you can preview the new style on the right hand side)
Click Save Settings
Themes without a white background are:
Grey
Friendly
Manni
Black
Fruity
Native
Beige
Perldoc
EDIT
I don't believe there is way to change the overall theme for BitBucket within their settings but you could use StyleBot (a Google Chrome plugin) to change the CSS. StyleBot instantly changes the CSS and persists your changes for future visits.
Best Answer
From the dashboard of being signed in, there will be sub-menus of:
Click on the "Pull requests" navigation menu item and then click on the "Mine" that appears on that page.
You'll see something like the following:
That page will list all of the pull requests you've created on Bitbucket.
If you can't see that as an option, you may need to turn that option on under your account settings.