UI-wise, I'd do it differently.
Currently, the description of what the checkbox affects and the checkbox itself are too far separated, plus it makes it sound like the layout is going to be changed even if the checkbox is not checked. Instead what I'd do is, below the general description paragraph (above the actual selection), put

[ ] Change the workspace layout to the one in the selected theme

unchecked by default, and if it's checked, show directly below
"Warning: When clicking "Apply", your current workspace layout (including all panels and desktop widgets) will be irrevocably lost and changed to the one defined in the Look & Feel theme."

In D2448#46454, @colomar wrote:

UI-wise, I'd do it differently.
Currently, the description of what the checkbox affects and the checkbox itself are too far separated, plus it makes it sound like the layout is going to be changed even if the checkbox is not checked. Instead what I'd do is, below the general description paragraph (above the actual selection), put

[ ] Change the workspace layout to the one in the selected theme

that will always happen, for how the architecture is done now i have no way to make it not switch the layout, neither i would have any idea how to make this configurable

the checkbox is between throwing away any change the user may have done or not