In Field editing mode, you'll see a list of all the fields in the right column.

If you don't see that, choose View > Show/Hide > Tools Pane.
Then use the following steps to select all the fields:
- Click on ONE of the fields to select it,
- press Ctrl-A (Win) / Cmd-A (Mac),
- Right-click on one of the fields and choose Properties and make the changes to Appearance
Note: This assumes you only have Text Fields in the form to edit.
If you only want to select all the text fields replace step 1 & 2 above with:
- Ctrl / Cmd-click on each text field in the Tools pane,
- Scroll down and continue selecting the fields to edit until you've selected all the text fields