1/ Setup submodule(s). After cloning opm-theme, get inside and:
    opm-theme (master)$ git submodule init
    Submodule 'bootstrap' ([email protected]:twbs/bootstrap.git) registered for path 'bootstrap'

    opm-theme (master)$ git submodule update
    Cloning into 'bootstrap'...
    [...]

2/ install node dependance and grunt. From your opm-theme folder:
    opm-theme (master)$ npm install .

   If grunt is not available on your system, you can install it using npm as well:
    opm-theme (master)$ npm install grunt-cli

3/ compile using grunt. If installed using npm in the repo, you will find it
   under "node_modules/grunt-cli/bin/grunt":
    opm-theme (master)$ node_modules/grunt-cli/bin/grunt
    Running "clean:dist" (clean) task
    Cleaning dist...OK

    Running "copy:fonts" (copy) task
    Copied 21 files

    Running "less:dev" (less) task
    File dist/css/opm-theme.css created.

    Running "less:minify" (less) task
    File dist/css/opm-theme.min.css created.
    Original: 7504 bytes.
    Minified: 6218 bytes.

    Running "concat:full" (concat) task
    File "dist/css/opm-theme.css" created.

    Running "concat:min" (concat) task
    File "dist/css/opm-theme.min.css" created.

    Done, without errors.

4/ Resulting files are produced in "dist/"