Previously, the only way to select all the module labels was with the
following kind of selector:
```css
.modules-left > widget > label,
.modules-center > widget > label,
.modules-right > widget > label {
/* ... */
}
```
(and a matching block for the `box` containers).
Now, this can be expressed as
```css
label.module, box.module {
/* ... */
}
```
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