If you want to use the built-in themes to customise the bubble count colours, then you can also make use of the "data-count-theme":
- <ul data-role="listview" data-count-theme="b">
A more complete example that includes colour formatting for some of the other attributes of a list would look like this:
- <ul data-role="listview" data-inset="true" data-theme="d" data-divider-theme="e" data-count-theme="b">
The advantage of your method is that you can really customise it to anything you want. However the built-in themes make the bubble text seem 'sharper'.