Unable to change current extjs theme in app

I have an already configurated ExtJS 5 who seems to have one workspace with one custom theme.

This custom theme (x-theme) is placed here:

extjs/x-workspace/packages/x-theme

And I am trying to change the theme from ext-theme-neptune to ext-theme-gray, here:

extjs/x-workspace/packages/x-theme/package.json

Where I have changed the following argument:

"extend": "ext-theme-neptune"

to

"extend": "ext-theme-gray"

After changing this, seeing no changes in webgui, I tried to re build doing,

sencha app refresh
sencha app build

in,

extjs/x-workspace/x

but got several errors and a big BUILD FAILED:

[INF] Processing Build Descriptor : default
[INF] Loading app json manifest...
[ERR] C2000: Rhino Parse Error (illegal character => Mac OS X           2??ATTR?xPuTTYThis resource fork intentionally left blank   ??) -- /home/user/x/static/extjs/x-workspace/x/app/model/production/._BenchModel.js:1:1
[ERR] C2000: Rhino Parse Error (Compilation produced 1 syntax errors. => null) -- /home/user/x/static/extjs/x-workspace/x/app/model/production/._BenchModel.js:1
[ERR]
[ERR] BUILD FAILED

From Sencha Documention for themes I cannot find anything more than I have already done.

Any of you with more experience can highlight me on how to change ExtJS theme?

Thanks in advance!

Note 1: I have no experience in ExtJS.

Note 2: There is no documentation left by the person who have setup ExtJS in first place.

UPDATE 1:

I was able to bypass those errors, but now I am getting the following:

[ERR] C2000: Rhino Parse Error (missing ; before statement =>
let record = me.up().record;) -- /home/user/x/static/extjs/x-workspace/x/app/view/production/JobFormDetails.js:130:23 [ERR] C2000: Rhino Parse Error (missing ; before statement =>
let message = '';) -- /home/user/x/static/extjs/x-workspace/x/app/view/production/JobFormDetails.js:131:24 [ERR] C2000: Rhino Parse Error (Compilation produced 2 syntax errors. => null) -- /home/user/x/static/extjs/x-workspace/x/app/view/production/JobFormDetails.js:1 [ERR] [ERR] BUILD FAILED

where in that JobFormDetails I have this part that has missing statements, but cannot figure what where is missing ';' :

listeners:{
    beforerender:function(me){
        'use strict';
        let record = me.up().record;
        let message = '';
        console.log(me);
        console.log('and this is record');
        console.log(me.up().record);
        console.log('this is record');
        console.log(record);
        if (record) {
            const recDate = new Date(record.date);
            const now = new Date();
            message = 'Job production is running';
            if (recDate > now) {
                message = 'Job scheduled to start on ' + record.date;
            }
            if (record.closed === true) {
                message = 'Job completed';
            }
            record.total_quantity_status = 'Total production: ' + record.quantity + ' parts';
        } else {
            record = {};
        }
        record.status = message;
        me.getForm().setValues(record);
    }
}

Note 3: I am not a JS expert.

It looks like you are doing everything right. But you have an error in the model file /home/user/x/static/extjs/x-workspace/x/app/model/production/._BenchModel.js in your application.