- Download the Solidity XML file or the alternative Solidity Plastic Code Wrap theme XML
- Open Notepad++
- Language menu > Define your language... > Import > Choose the XML file you just downloaded > Should say "Import successful"
- Close the User Defined Language menu, restart Notepad++, reopen your .sol files
If the import fails and you have Notepad++ > v7.6, also try the method described here.