You change the theme (typically black) and the text in the Ubuntu software centre (in my case under Xubuntu) turns to be no more visible or readable only when selected.
The problem comes from text having the same background of the inside of that window.
To solve the problem we have to change the color from this CSS. /usr/share/software-center/ui/gtk3/css/softwarecenter.css
sudo nano /usr/share/software-center/ui/gtk3/css/softwarecenter.css
and change the first 2 lines from
@define-color light-aubergine #DED7DB; @define-color super-light-aubergine #F4F1F3;
to something like
@define-color light-aubergine #fff; @define-color super-light-aubergine #333;
Or use Synaptic so you could also get the benefit of getting rid of that commercial stuff in the Ubuntu software centre.