When I first tried the solarized theme, I had a similar problem with the helm menu as shown above. The highlight on diff and magit was even worse.

I started trying to fix it myself, but in the end I realized that there are at least three different versions of the solarized theme in MELPA. The one called solarized-theme is the one you want. The helmet selection panel is very thin and underlined, as shown below.

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