yeah scripts dont run on load like this, thats why i wrap my scripts in the onload of an img object
code = """<img style='display:none' src='/favicon.ico' onload=\"
console.log('hey');
\"></img>""".replace("\n", "").replace("\t", "")
return code
Do note that the added a css sheet will not disspear, when closing the view, and if you open a new view a second css sheet will be made... (i think)
Css sheets also can get cached, so this might turn out ugly after a while.
Its better to just add in a <style> object instead of a script (or the onload img) for css related injections. and use python to generate your style "dynamicaly"
styles=".iaDropdownCommon_options [data-label='YourLabel']:before { content: url('/system/images/Builtin/icons/16/check2.png'); }"
code = """<style>
"""+styles+"""
\"></style>""".replace("\n", "").replace("\t", "")
return code