In Safari, press COMMAND-F to find some text. Once the text is found, it will be highlighted. If the Find window is still open, the text will be highlighted grey, otherwise it'll be highlighted in whatever colour you have picked in the Appearance pane of System Preferences.
However, if you press COMMAND-F, enter your text, press RETURN to hide the Find window, you can find the next selection of the same text by pressing COMMAND-G. All of these will be highlighted in your chosen colour (not grey) because the browser window is the foremost window. 🙂