We may need to work on this a bit more.
If you can link me to a series of good viable themes you might want to use, excluding the Mint-X series as I have copies of those, let me see if I can help narrow the visual border while increasing its grab area on a theme that is more conducive to your needs.
A short post in a net thread is likely just not enough to convey what is needed.
As a theme creator myself or @Storm can say that theming takes quite a bit of practice and a lot of checking of what works and what does not work on a case-by-case basis depending on the other included elements within a theme.
We all know you can do this and you can even continue forward to making your own full themes if you want.
But it is asking too much of anyone that they do it in One Day, learning all that is needed in that day.
Alternatively, I can create a theme that looks much like Mint-X-Sand, but is not Mint-X-Sand but distinct in its elements, while visually mimicking Mint-X-Sand. In that way, it may look slightly different in some subtle ways, but more easily attain your goal - a thin looking border with lots of grab area - without trying to force fit that into the existing Mint-X theme.
And I could send it to you whole and complete without you needing to do anything more than place it in your folder and select it from the settings.
Let me know.