Error during startup when using a non-default theme

[mcr-paulanand]

[reckart]

@solomax There was this PR to use Webjars for the JQuery UI themes - it looks like, we don't use webjars though and you have added a script to update the themes. Did using the webjars not work?

[solomax]

Hello @reckart,

from what I can see in these commits/PRs (to be fair I totally forgot about these changes I've made ~2 years ago :)))

This is not about loading JQuery themes via WebJars, but about fixing theme switching
Due to JQueryUI itself is now being loaded via WebJars theme initializers need to be updated :))