Ok, let's add a 1px stroke #F5F5F5 in between the theme image and the info block below:

Because of the frame-less way themes are displayed in Disco Pane, light-colored, translucent, or border-less themes are avoided because they appear to just float in space.
For example, consider the awkwardness of a theme like this in Disco:
https://addons.mozilla.org/firefox/addon/cat-glasses/
... should we consider addressing in some way? Apply some type of border?
@pwalm, @designakt
We're in Taipei this week, but will look at it when we get back home.
I'm not sure if adding a border saves this particular theme. The theme looks great when applied, but that big white block under the cat is going to look weird, no matter what:

I'm ok with this theme looking a bit "off" in relation to the other themes (since it looks awesome in action).
This particular theme was just meant as an example. The issue crops up pretty regularly with other white-ish or translucent themes. If interested try experimenting with other light-colored themes and see what you think.
I filed this in response to @designakt requesting we select more Disco themes that do NOT invert the color of the Firefox UI buttons. But because I tend to select darker colored themes for Disco (due to aforementioned issue) nearly all of them invert the buttons for contrast.
@devaneymoz Talking with @pwalm I think he sees no problem with bright themes as there is a shadow around the themes card that serves as a border. @pwalm correct me if I got that wrong.
Ok, let's add a 1px stroke @ #F5F5F5 in between the theme image and the info block below:

@tofumatt or @mstriemer: Can I ask one of you guys to please add this to your (very long) to-do list? 馃槄 Thanks!
If this is your first contribution, please refer to https://wiki.mozilla.org/Add-ons/Contribute/Code on how to get started.
Mentor: @tofumatt
Is there a way for us to check this? Because, in -dev, Disco Pane, we do not have "white" themes available so...need help. Thanks!
@ValentinaPC easiest way is to delete the src attribute with dev tools. This will break the image and it should just show alt text.
Verified this on AMO-dev FF55(Win 7).
Postfix screenshot:

Most helpful comment
I'm not sure if adding a border saves this particular theme. The theme looks great when applied, but that big white block under the cat is going to look weird, no matter what:

I'm ok with this theme looking a bit "off" in relation to the other themes (since it looks awesome in action).