File->Settings->Editor->Colors & Fonts-> In scheme name select Darcula and apply to see a awesome dark background theme editor

Android Studio 3.1.2 File->Settings->Editor->Color Scheme-> In scheme name select Darcula and apply to see a awesome dark background theme editor

Windows: Ctrl + Alt + L

Linux: Ctrl + Shift + Alt + L

macOS: Option + Command + L

Reference: Key Commands and here are all of the commands for Windows/ Linux users and for Mac users.

As Rohit faced a problem in Ubuntu with the format code shortcut, this is due to the Ctrl + Alt + L key being used to lock the screen in Ubuntu.

I found that Ubuntu handles this keyboard shortcut first. So you should bind the Ctrl + Alt + L keyboard shortcut to something else so that it doesn't conflict with Ubuntu.

Steps

1. Go to System Tools → System Settings → Keyboard → Shortcuts tab → System → Lock Screen.

2. Select the row New Accelerator..., then press any special key with the Alpha key (e.g. Shift + L). You should've successfully changed the keyboard shortcut.

3. Check if the keyboard shortcut now works in Android Studio.

Alternative method

Ctrl + Alt + Shift + L (to show a dialog)

You can also use Eclipse shortcuts.

• Windows and Linux: Go to menu File → Settings → Keymap
• macOS: Go to menu Preferences → Keymap and choose Eclipse from the drop-down menu.

Using MenuBar

• Select code which you want to reformat

• Click on Code in the menu bar and select Reformat Code as shown in the image below:

  

In Android studio 3.5, there is problem with rearranging the xml code, it also rearrange the views as well, so need to use these settings first.

Settings -> Editor -> Code Style -> XML-> Set From -> Predefined Style > Android.

Update for Android Studio 4+

Since Android Studio 4+, you might face issue shortcuts on mac. Shortcuts not working after updating to Android studio 4, in Mac? have an answer for that.