Yes, you are already using the settings with the most contrast.
But have you somehow changed the font that is used? It looks quite different from what I would expect to see. The font is super thin, but the bold item is super bold.
For comparison, here is what it looks like using your chosen background image in a standard osTicket Awesome installation:
Obviously, it still isn’t ideal as the background doesn’t provide a lot of consistent contrast. But it looks better than what you have.
You can add the following to /osta/user-styles.css:
#header a {
font-weight: 400;
}
That will make the font of all menu items bold, rather than just the current item.
If you still aren’t happy with that, you should consider turning off Header Background > Use Backdrop Image