I believe I have found a fix for getting the link preview to become legible -- even if it doesn't enable full control.
I started out using the Google Chrome Theme Creator found here:
(Note the final string tacked onto the end because you'll find that folder in your extensions folder -- at least I did.
Once installed and before using it, you'll need to change the way that it generates the theme. As far as I know, there is no way to re-import a theme once it has been installed. That would be lovely and was what I tried to find first. However, failing that I found out that I could change their theme generation code to suit my own purposes.
Here is the full path to the file I ended up modifying (Windows 7):
In the manifest.json file for my theme I created 'out of the box', I noticed that there was an opacity applied to the "toolbar" colors which was not specified in themes I downloaded elsewhere. It was set to 0.6 and I couldn't find a way to tell Chrome to re-interpret this file with my change. This opacity ended up making the font extremely hard to read and led me to your question. If you make this change to the My Chrome Theme app, you may no longer hope for a way to control the font since it will again be legible.
In main.js, I searched for 0.6 and immediately found only one reference in all of the minified code. The line appears like this:
Change that 0.6 to 1 (or somewhere in-between to your liking) and I think you will be well on your way to having a satisfactory link-hover-preview.
You may not have been using this theme generator, but I think you may be happy with the results after changing main.js.