How to highlight all search results within the Terminal application screen buffer
Cmd+F to open the find window, and then type your find keyword
After you've entered your keyword, go to Edit → Find → Select All
Your search term should now be highlighted:




Unfortunately, this has no keyboard shortcut by default and you can't add a keyboard shortcut for this via the normal Mac settings (without installing a third-party automation app) because it would conflict with the "Select All" option that is being used as a top-level action inside the Edit menu, before the Find submenu.

However, you could add a keyboard shortcut for "Select All in Selection", the only downside to this is you have to select the region you want to highlight before executing the key combination. Here's how to set up this workflow:


Click on the  menu on the top-left and go to System Preferences
Select Keyboard.
Select the "Shortcuts" tab
Click on "App Shortcuts" on the lefthand side menu
Click on the "+" button, then select "Terminal.app" as the app, "Select All in Selection" in the Menu Title field, and the "Shortcut" key combination you want to use (Cmd+Shift+F in my case, but you can choose whatever you want).
Click "Add"




Now, inside Terminal.app you can use this like:


"Cmd-A" to select all
"Cmd-F" to open the find bar
Type in your search phrase
Because everything is still selected, you can use your shortcut you just specified (e.g. Cmd+Shift+F) to highlight your find keyword in your terminal window.