I was looking for this functionality myself, so I wrote this pair of regex search-and-replace based macros for Notepad++. Format CSS formats a CSS document while Minify CSS makes it smaller.
To install them, locate your shortcuts.xml file, usually at
C:\Users\%username%\AppData\Roaming\Notepad++\shortcuts.xml
and paste the whole block into the <NotepadPlus><Macros>
block.
You need to use another editor than Notepad++, OR close Notepad++ by killing the process after you edit this file. Otherwise, Notepad++ will overwrite the file on close.
After installing, you can run those macros from the Macros menu in Notepad++, as well as rename them or assign hotkeys using Edit shortcut/delete macro.
In case someone finds a css for which this script doesn't work, please make a paste and post the link in the comments so I can look at it.
Macro source XML
<Macro name="Format CSS" Ctrl="no" Alt="no" Shift="no" Key="0">
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="([^\r\n;])\}(?![\r\n])" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="\1;\r\n}\r\n" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="([^\r\n])\}(?![\r\n])" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="\1\r\n}\r\n" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="([^\r\n])\{(?![\r\n])" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="\1\r\n{\r\n" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam=";(?![\r\n])" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=";\r\n" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="(^(?![ \t]).+;)" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=" \1" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="\}(?!(\r?\n\r?\n))" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="}\r\n" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam=":(?![ ])(?=.+;)" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=": " />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
</Macro>
<Macro name="Minify CSS" Ctrl="no" Alt="no" Shift="no" Key="0">
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="[\r\n\t ]+" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=" " />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="[ ]*\}[ ]*" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="}" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="[ ]*\{[ ]*" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="{" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="[ ]*;[ ]*" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=";" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam=";\}" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam="}" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
<Action type="3" message="1700" wParam="0" lParam="0" sParam="" />
<Action type="3" message="1601" wParam="0" lParam="0" sParam="[ ]*:[ ]*" />
<Action type="3" message="1625" wParam="0" lParam="2" sParam="" />
<Action type="3" message="1602" wParam="0" lParam="0" sParam=":" />
<Action type="3" message="1702" wParam="0" lParam="768" sParam="" />
<Action type="3" message="1701" wParam="0" lParam="1609" sParam="" />
</Macro>