This could depend on the SQL Developer release you have installed and how the highlighting was done, so we need more information.
Did you drag the mouse over the grid or right-click in the grid and select Find / Highlight from the context menu? If just dragging, then look at Tools > Preferences > Environment > Look and Feel. Prior to the 4.0.x releases, you could also choose a Theme. If changing the Look and Feel / Theme does not help, then possibly the user settings got corrupted and you may have to reset them. If 4.0.x, you can try doing that via the Window > Reset Windows to Factory Settings, but in 3.2.x and earlier must delete the relevant settings file (or, more simply, the entire user settings directory structure for the particular release number in use).
Using Find / Highlight, you can choose a highlight color from a palette and persist different colors for distinct search strings, enable / disable highlighting for a specific search string, persist the highlights or clear them -- fun to play with, I suppose. Clearing persisted highlights is possible either via the same context menu or Tools > Preferences > Database > Object Viewer > Clear button.
Hope this helps,
Thanks for the response, and sorry I forgot to mention my version. I'm using 188.8.131.52.
I have used the Find/Highlight, and that suggestion is helpful, but not exactly what I'm asking about. I was asking about when you click on the far left of a row is a query result, on the row number indicator, and the whole row gets selected/highlighted.
I changed the look and feel to Oracle from Mac OSx and that was helpful, from the point of view that the row highlighting was infinitely more readable using the Oracle look and feel. Thanks again for the response and pointer.