The first thing to consider is using a text highlight at all. They nearly always don’t look very good. But as you can see, a straight line renders out OK as a highlight.
If you really must have a text highlight, consider using a simpler font. Simple sans-serif fonts will look better with a button highlight than more complex fonts.
If you really must use that particular font, then consider creating two subpicture layers: a stroked subpicture layer and another subpicture layer for the inner part of the stroke. Set the color of the stroke to a color close to your menu background color, with less opacity than the inner part of the stroke.
If that still doesn’t give you what you want, consider using a layered menu (which must be built by hand in Encore with buttons set to auto-activate that link to duplicate menus that have the “highlight” you want with no subpicture layer restrictions on colors/opacities). But the caveat is this is a lot of manual work linking all the menus, and that these menus will not behave as expected in a software DVD: you must click the button to advance to the next menu showing your button state.
Myself, I tend to go for the simplest option, which is what you’ll find on most Hollywood DVDs.