I created a custom OOUI theme by the way that explained in https://www.mediawiki.org/wiki/OOUI/Themes#Custom_themes and #Using themes in MediaWiki and it works in MW 1.34 with VE.
But in 1.35.0-rc.0 and in 1.35.0-rc.1, more strictly say, after b2ce2db332 (gerrit link), all JavaScript breaks in all skins I installed in my local wiki.
Browser console log:
Main_Page:11 GET http://localhost:8080/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector net::ERR_ABORTED 500 (Internal Server Error)
Accessing http://localhost:8080/load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector will result below outputs:
[58ee199e0b429316fb07e3a5] /load.php?lang=en&modules=startup&only=scripts&raw=1&skin=vector TypeError from line 112 of /var/www/html/includes/resourceloader/ResourceLoaderOOUIImageModule.php: Return value of ResourceLoaderOOUIImageModule::loadOOUIDefinition() must be of the type array, boolean returned
Backtrace:
#0 /var/www/html/includes/resourceloader/ResourceLoaderOOUIImageModule.php(43): ResourceLoaderOOUIImageModule->loadOOUIDefinition(string, string)
#1 /var/www/html/includes/resourceloader/ResourceLoaderImageModule.php(420): ResourceLoaderOOUIImageModule->loadFromDefinition()
#2 /var/www/html/includes/resourceloader/ResourceLoaderModule.php(823): ResourceLoaderImageModule->getDefinitionSummary(ResourceLoaderContext)
#3 /var/www/html/includes/resourceloader/ResourceLoaderStartUpModule.php(209): ResourceLoaderModule->getVersionHash(ResourceLoaderContext)
#4 /var/www/html/includes/resourceloader/ResourceLoaderStartUpModule.php(405): ResourceLoaderStartUpModule->getModuleRegistrations(ResourceLoaderContext)
#5 /var/www/html/includes/resourceloader/ResourceLoaderModule.php(717): ResourceLoaderStartUpModule->getScript(ResourceLoaderContext)
#6 /var/www/html/includes/resourceloader/ResourceLoaderModule.php(685): ResourceLoaderModule->buildContent(ResourceLoaderContext)
#7 /var/www/html/includes/resourceloader/ResourceLoaderModule.php(820): ResourceLoaderModule->getModuleContent(ResourceLoaderContext)
#8 /var/www/html/includes/resourceloader/ResourceLoader.php(775): ResourceLoaderModule->getVersionHash(ResourceLoaderContext)
#9 [internal function]: ResourceLoader->{closure}(string)
#10 /var/www/html/includes/resourceloader/ResourceLoader.php(787): array_map(Closure, array)
#11 /var/www/html/includes/resourceloader/ResourceLoader.php(874): ResourceLoader->getCombinedVersion(ResourceLoaderContext, array)
#12 /var/www/html/load.php(51): ResourceLoader->respond(ResourceLoaderContext)
#13 /var/www/html/load.php(38): wfLoadMain()
#14 {main}After investigating, I found ResourceLoader errored when trying to load extensions/VisualEditor/modules/ve-mw/ui/styles/<My_Custom_OOUI_Theme_Name>/ve.ui.Icons.json that does not exist of course.
Removing VE or checking out before "Add some typehints" commit or removing "OOUIThemePaths" and "SkinOOUIThemes" in skin.json makes the error to disappear.