Apply custom background only to top level controls widget

Great launcher you have here. Glad to see someone picked up the slack for Paradox. :)

One problem I'm having though: currently the custom background is inherited by all children of "widget", which can make things hard to see on some styles, e.g:

![2013-02-23-070145_699x551_scrot](https://f.cloud.github.com/assets/65656/188037/72686e54-7dba-11e2-9012-62502321f7f6.png)

This commit makes the background only apply to widget, which matches the Windows launcher:

![2013-02-23-070659_699x551_scrot](https://f.cloud.github.com/assets/65656/188039/883263d4-7dba-11e2-888e-4b25da2cf49b.png)