Problem running Gulp/Stylus from Bitbucket Pipeline

[wallmanderco-jameswills]

I am a complete newbie on Bitbucket Pipelines, but I am just looking to create a simple process so that just before files are pushed, the Gulp process is run and compiles all required assets before the push, which then uploads the files to our server via a webhook. I currently have the below:

pipelines: default: 
    - step: script: 
        - cd wp-content/themes/my-wp-theme 
        - npm install 
        - npm install -g gulp 
        - npm install stylus -g 
        - gulp

However the above is failing with the following:

Error: 'tried to require: stylus'.None found. Make sure one has been installed! 

How can I get around this error? Also, the pipeline takes 2-3 mins to run! Is there any way to speed this up?