You got 2 options
a) for regular GUIs a combination of halos and inspector will give the information you need about the proper location, size etc and allow you to fine tune your morphs far easier than doing it by code
b) for custom GUIs you divert the design to a proper design application like gimp , photoshop or blender and create the look you want with the elegance of custom design, you can take a look on my ChronosManager project at Catalog browser as an example of how you can import images with transparency
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NON PHARO OPTIONS
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c) this option I have not tried but it should be possible to design also a custom GUI using a vector designer , like adobe illustrator or Inkscape and import those vectors as svgs formats back to Pharo. Athens has an SVG importer and so does Roassal.
d) also you can use HTML and Seaside/PharoJS and leverage well know technologies like js dom/html5/CSS as you may be aware there are a ton of tools out there that design GUIs using these technologies
e) you could use my Python bridge to use PyQT that uses QT probably the most powerful GUI out there that also come with a super powerful GUI designer called QTCreator which has evolved it full blown IDE, this one requires knowledge of Python and you may have to modify the bridge to fit your needs
f) if you are not in a hurry I am making a Pharo to Unreal bridge which gives access not only 2D GUIs but also 3D GUIs with the graphic capabilities of AAA games and the added bonus of Virtual Reality GUIs if you want to blow the minds of your users.
I could carry on but I think you can figure out the rest by yourself.