Hi,
I tried following the instructions from the "training" website to change some plone-font-weight-* values.
I copied the Barceloneta theme to a test theme, edited there the variables.plone.less file, saved changes, selected the file barceloneta.plone.less then clicked on "Tools > Build CSS" then I got a message saying "Working..." (and citing barceloneta-compiled.css as target file) but nothing changes, the message keeps displaying with no apparent activity and barceloneta-compiled.css contents do not change.
Last night I didn't check immediately but after 2 to 3 trials the console was empty. So I just tried anew and I have some more information to give:
First, after clicking on "Build CSS" this time I had to click on a "Compile" button to launch the process ; I think I got it once last night but on subsequent tries, as soon as I clicked on "Build CSS" the "Working..." message appeared, as it does now - so it seems the "Compile" button does not show up but at the first try (the server rebooted this morning, incidentaly).
This is what I had in Chromium console (Firefox was much less verbose) :
thememapper-compiled.js:40581 [Violation] Avoid using document.write(). https://developers.google.com/web/updates/2016/08/removing-document-write
init @ thememapper-compiled.js:40581
window.IFrame @ thememapper-compiled.js:40518
showLessBuilder @ thememapper-compiled.js:40735
(anonymous) @ thememapper-compiled.js:40657
dispatch @ default.js:6
t.dispatch @ thememapper-compiled.js:34895
r.handle @ default.js:6
VM218 less.js:406 [Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
doXHR @ VM218 less.js:406
loadFile @ VM218 less.js:465
loadStyleSheet @ VM218 less.js:617
loadStyleSheets @ VM218 less.js:628
(anonymous) @ VM218 less.js:700
less.refresh @ VM218 less.js:696
(anonymous) @ VM218 less.js:79
Promise.then (async)
2../add-default-options @ VM218 less.js:77
s @ VM218 less.js:13
e @ VM218 less.js:13
(anonymous) @ VM218 less.js:13
(anonymous) @ VM218 less.js:13
(anonymous) @ VM218 less.js:13
default.js:7 POST https://intranet.XXXXXXXXXXXXX/intranet/++theme++XXXXXXXXXX/@@plone.resourceeditor.filemanager-actions 500 (Internal Server Error)
send @ default.js:7
ajax @ default.js:7
doAction @ thememapper-compiled.js:39715
saveThemeCSS @ thememapper-compiled.js:41297
(anonymous) @ thememapper-compiled.js:40777
Promise.then (async)
onLoad @ thememapper-compiled.js:40765
load @ thememapper-compiled.js:40608
onload @ @@theming-controlpanel-mapper?_authenticator=e2b869920c624294ac6728173dc9c25dde97116b:1
load (async)
init @ thememapper-compiled.js:40581
window.IFrame @ thememapper-compiled.js:40518
showLessBuilder @ thememapper-compiled.js:40735
(anonymous) @ thememapper-compiled.js:40657
dispatch @ default.js:6
t.dispatch @ thememapper-compiled.js:34895
r.handle @ default.js:6
I'll try to reproduce it on a fresh install ; this Plone instance is 10 years old and has been migrated from 4.3 to 5.1 last year. It might will convey some oddities...