And the answer is very simple: You don't need to open the file on your computer! Just transfer it to your phone.
In ase you want to open the file (which doesn't make much sense) go to forum.nokia.com and search for Carbide.ui, which is the official theme creation tool. It can open theme files. But then again, there's no need to open the file on your computer. ANd lasst but not least, you must register at forum.nokia.com before downloading. It's free anyway.
And that hint is valid for any situation when the file is "unknown". You musr find a program that lets you open the file. In this case, it's Carbide.ui. Other cases may depend on the file type.
See if this solves the blue bars issue. That topic is about S40 themes, but maybe the solution will work for S60 themes. I can't check because I don't have a Symbian device.
One thing is how much the device officially supports (the amnufacturer says how much the device supports safely) and the other is how much you can put.
The manufacturer says the device supports up to X Gb. That means that's the maximum reccommended is X Gb. In most cases you can use a bigger capacity card, but then the device may slow down, hang or take ages to load the contents, especially when accessing playlists or music files.
GSMArena claims they have used a 16 GB card. They have been around for long enough to know what safe use means. I guess if they say they can use a 16 GB card, it means they didn't notice significant slowdowns or freezes.
But in any case, whenever you use a card bigger than the recommended by the manufacturer, you are using it at your own risk.
They are very similar in gfeatures. The W380 has a sensor which allows you to mute calls or snooze the alarm by waving your hand in front of the camera.