Unfortunately Oracle doesn't make it easy to import a new colour scheme into SQL Developer, thus a little bit of hacking is required.
Close SQL Developer. This is important. If you modify the scheme file while SQL Developer is open, your changes won't be saved.
dtcache.xml in the SQL Developer's settings directory.
Linux or Mac OS X:
<schemeMap>tag inside dtcache.xml file. Insert the content of the color scheme xml file inside
<schemeMap>alongside the other colour schemes. Be careful not to break the XML.
Launch SQL Developer. Navigate to menu Tools->Preferences, then select item Code Editor -> PL/SQL Syntax Colors in the left pane.
Select theme in the "Scheme" drop down list on the top.
See original instructions providing by Ozmoroz
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