You may have noticed the Edit -> Find... dialog (Ctrl-F in a worksheet or editor) contains the option Highlight All Occurrences. If that option is not grayed out and you can actually check it off, then it will do what you want. Highlighting a word before opening the Find dialog automatically makes it the Text to Search For.
Unfortunately Highlight All Occurrences seems to be grayed out in all current or older SQL Developer releases I have seen. I believe it is fixed in the next release.