atom-cssbeautify 
Beautify your CSS using CSSBeautify.
Installation
$ apm install cssbeautify
Usage
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Packages>CSSBeautify>Beautify CSS - Context Menu >
Beautify CSS
Selected text will be beautified if there is selection.
Configuration
Execute on Save
If you want to apply beautifying when you save, check it on.
Indent Type
Choose the indent type from space (default) or tab.
Indent Size
Set the indent size (default is 2). This value is used when you are choosing space for indent type.
Open Brace
defines the placement of open curly brace, either end-of-line (default) or separate-line.
Auto Semicolon
always inserts a semicolon after the last ruleset (default is false).