You can also define highlighted text as a "character style" .
1) Select some text in your document
2) Click on the "a" with the red cross behind in in your font toolbar and choose the color you'd like to use as highlighted text for your document (yellow for me)
3) go to View>Show Styles Drawer
4) select the highlighted text once again
5) under "character style" to the right of "none" there should be an arrow. click that and select "create new character style from selection"
now whenever you want to highlight text, just select the text and click on the new character style you've created
you can also add a hotkey to your new character style by selecting it in the styles drawer, click on the arrow and under "hot keys" choosing the desired key