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I am not sure how restrictive it is, but you have bower 1.8.8 installed globally. Maybe you need to install a 1.3 version? Just guessing here. If that works, you could try the same with gulp and yo.  At least for gulp and yo the major version changed.

Thanks ChristophI have a strange behaviour.
Following the step described below I was able to have a generator -liferay-theme installed without errors.But the theme generation gives an error on sass and build failsI'm using Ubuntu 18.04.3. The username is sysop and the home directory is /home/sysopI created a .npmrc file in /home/sysop containing

prefix=/home/sysop/.npm-packages

I added in /home/sysop/.profile these lines

NPM_PACKAGES=/home/sysop/.npm-packages

PATH=${PATH}:${NPM_PACKAGES}/bin

Then I installed npm distribuited with the OS

sudo apt install npm

After various attempts (I'm using a virtual machine; so I do an attempt then revert to a snapshot) I discovered that module have to be installed globally and locally.In this situation the generator -liferay-theme can be installed locally without errors (stil installing globally gives the three unmet dependencies)I worked inside a directory named /home/sysop/temiThe commands are:

cd ~/temi

npm install -g yo

npm install -g gulp

npm install -g bower

npm install -g node-gyp

npm install -g pangyp

npm install -g node-sass

npm install yo

npm install gulp

npm install bower

npm install node-gyp

npm install pangyp

npm install node-sass

npm install generator-liferay-theme@7.0.6

Now I can generate a theme; but there is a sass error. The relevant lines are:

sysop@vocsem:~/temi$ yo liferay-theme

     _-----_
    |       |    .--------------------------.
    |--(o)--|    |  Welcome to the splendid |
   `---------´   | Liferay Theme generator! |
    ( _´U`_ )    '--------------------------'
    /___A___\    
     |  ~  |     
   __'.___.'__   
 ´   `  |° ´ Y ` 

? What would you like to call your theme? Base MSB
? Would you like to use this as the themeId? base-msb
? Which version of Liferay is this theme for? 7.0
? Do you need Compass support? (requires Ruby and the Sass gem to be installed) No
   create package.json
   create bower.json
   create .gitignore
   create gulpfile.js
   create src/css/custom.css
   create src/META-INF/context.xml
   create src/WEB-INF/liferay-plugin-package.properties
   create src/WEB-INF/src/resources-importer/readme.txt
   create src/WEB-INF/src/resources-importer/sitemap.json

I'm all done. Running npm install & bower install for you to install the required dependencies. If this fails, try running the command yourself.
[snip]

npm WARN prefer global pangyp@2.3.3 should be installed with -g

> node-sass@2.1.1 install /home/sysop/temi/base-msb-theme/node_modules/node-sass
> node scripts/install.js

Can not download file from https://raw.githubusercontent.com/sass/node-sass-binaries/v2.1.1/linux-x64-node-8.10/binding.node

> node-sass@2.1.1 postinstall /home/sysop/temi/base-msb-theme/node_modules/node-sass
> node scripts/build.js

module.js:549
    throw err;
    ^

Error: Cannot find module '/home/sysop/temi/base-msb-theme/node_modules/node-sass/node_modules/pangyp/bin/node-gyp'
    at Function.Module._resolveFilename (module.js:547:15)
    at Function.Module._load (module.js:474:25)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:188:16)
    at bootstrap_node.js:609:3
Build failed
base-msb@0.0.0 /home/sysop/temi/base-msb-theme
├── async@0.9.2 
├─┬ c[email=mailto:check-source-formatting@1.1.10]heck-source-formatting@1.1.10 [/email]

[snip]

I'm confused: I don't know what is missing.Any hint?

Well ,
I discovered that that node-sass is broken at the global level because of a missing bower

sysop@vocsem:~/temi$ npm list|grep sass
├─┬ node-sass@4.13.0
│ ├─┬ sass-graph@2.2.4
sysop@vocsem:~/temi$ npm list --global|grep sass
├─┬ node-sass@4.13.0
│ ├─┬ sass-graph@2.2.4
npm ERR! peer dep missing: bower@>=1.3.0, required by generator-liferay-theme@7.0.6
sysop@vocsem:~/temi$

So seems that generator-liferay-theme ignores local modules
Also strange is that trying to reinstall globally bower seems installing correctly but the above command (node list --global) gives a different error

sysop@vocsem:~/temi$ npm install -g bower
npm WARN deprecated bower@1.8.8: We don't recommend using Bower for new projects. Please consider Yarn and Webpack or Parcel. You can read how to migrate legacy project here: https://bower.io/blog/2017/how-to-migrate-away-from-bower/
/home/sysop/.npm-packages/bin/bower -> /home/sysop/.npm-packages/lib/node_modules/bower/bin/bower
/home/sysop/.npm-packages/lib
└── bower@1.8.8 

sysop@vocsem:~/temi$ node list --global|grep sass
module.js:549
    throw err;
    ^

Error: Cannot find module '/home/sysop/temi/list'
    at Function.Module._resolveFilename (module.js:547:15)
    at Function.Module._load (module.js:474:25)
    at Function.Module.runMain (module.js:693:10)
    at startup (bootstrap_node.js:188:16)
    at bootstrap_node.js:609:3
sysop@vocsem:~/temi$

Any suggestion to solve this nightmare?
Can generator-liferay-theme be forced to use local modules?

UPDATETried another clean install using only global installationgenerator-liferay-theme gives the three unmet peer dependency; but "yo liferay theme" tries (and fail) to install what it needs
Attached here a file containing the versions og the global module installed and the complete transcripf of "yo liferay-theme"