Many thanks to Ozmoroz for providing clear instructions (Source):
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Close SQL Developer. This is important. If you modify the scheme file while SQL Developer is open, your changes won't be saved.
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Locate file
dtcache.xml
in the SQL Developer's settings directory. On my system it is located in directoryC:\Users\sergey\AppData\Roaming\SQL Developer\system4.0.3.16.84\o.ide.12.1.3.2.41.140908.1359
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Locate
<schemeMap>
tag inside dtcache.xml file. Insert the content of color scheme xml file inside<schemeMap>
alongside the other colour schemes. Be careful not to break the XML.
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Launch SQL Developer. Navigate to menu Tools->Preferences, then select item Code Editor -> PL/SQL Syntax Colours in the left pane.
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Select the color scheme name in the "Scheme" drop down list on the top.