Custom styling an Ext.tab.Tab component

I'm trying to style a Tab, so the background color is orange. I'm using the Sencha SDK to achieve this. I've gotten the styling to work, but if the tab is closable, that's when I run into issues, as I get a warning when I use `sencha app build`. The warning is:

HTML Code:

WARNING: @theme-background-image: Theme image not found: images/tab/tab-orange-close.png  
          on line 18 of Theming-workspace/build/temp/production/CustomTheming/slicer-temp/../../../../../ext/packages/ext-theme-base/sass/etc/mixins/theme-background-image.scss  
          from line 6 of Theming-workspace/build/temp/production/CustomTheming/slicer-temp/../../../../../packages/custom-theme/sass/src/tab/Tab.scss  
          from line 810 of Theming-workspace/build/temp/production/CustomTheming/slicer-temp/CustomTheming-example.scss

My tab config looks like this:

Code:

    items: [{
      title: 'Blah',
      tabConfig: {
        ui: 'orange'
      },
      closable: true
    }

My src/tab/Tab.scss looks like this:

Code:

    @include extjs-tab-ui(
      $ui: $ui-tab2,
    
      $ui-background-color: $ui-tab2-background-color,
      $ui-background-color-over: $ui-tab2-background-color-over
    );

And my var/tab/Tab.scss looks like this:

Code:

    $ui-tab2: 'orange';
    $orange: mix(red, yellow);
    $ui-tab2-background-color: mix(#fff, $orange, 20%);
    $ui-tab2-background-color-over: $orange;

Like I said, the orange styling shows up just fine. The problem is the closable icon not appearing, and that's what the warning is trying to tell me, but I'm not sure how I can create an image for this. Do I have to somehow use the slicer?