I'm using the new mint-y dark theme.
For some specific application (thundermail for example), I would like to use the light theme instead.
Is there a way to do so ?
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In general, this is not possible.
The Desktop Environment (DE), Cinnamon in your case, is only able to do things like draw a window, draw a button, decorate everything, stack windows, and all the other things you would expect to do with windows. It does not keep track of individual applications.
The individual applications just make requests to the DE to draw the various items: windows, buttons, etc. The applications are not aware of what DE they are making these requests to.
Your theme is a configuration setting inside the DE.
At the start I said "in general". There are applications that handle their own drawing of the user interface: all the window stuff. They do not hand this task off to the DE. They are any applications that when you run them, they look off. Most distributions go to great lengths to make everything look seamless. In the LM distro, this is accomplished with the introductions of X-Apps.
Some additional reading: