Image taken from this forum post.
This is problematic as it mandates solid colour in the interior of the rendered sprite. If I want a gradient, for example, I have no way (that I know of) to increase the vertical width or horizontal height of that inner cross. I've searched the stye sheets from base game and numerous mods but not seen any properties for setting the dimensions of that inner cross.
To solve this issue, I'd like an inner_size property adding to composition style sheets. For example, if I wanted my composition to be based on this image (a black button with a red indicator on the left):
I'd set corner_size = {9,4} and inner_size = {1,25} (the horizontal height of the cross would be 25 px, enabling me to get a chunk of gradient that will stretch over the main part of the button when rendered in game).
Note: If no inner_size is specified, it should default to {1,1} for backwards compatibility.
As there is already code to stretch sprites to arbitrary dimensions (eg. how the monolith sprites work) hopefully this request will be feasible. It will make it significantly easier to add subtle gradients to UI elements without resorting to fixed size monolith sprites or clunky grid (table) arrangements of buttons where each button has a monolith grabbed from the source image (yeuch!).