Yep, you can apply the outline shader to just about anything! I do also recommend you try playing around with Ren'py's built in outlines to see if you can get the effect you want, as the outline shader will be somewhat similar; you can apply outlines with decreasing transparency and increasing size through the outlines text property. Notably your example image is more a blobish suggestion behind each letter, whereas the outline shader (and built in text outlines) will be more like *glowing*. You might also be interested in my gradient shader tool if you want more blended gradients generally.