You can compile your own custom SASS to include only the CSS elements you use:
But Sencha's minified code shouldn't be as much of a problem as the debug versions if you are worried about load-time/memory usage.
//$tabs-dark: darken(desaturate($base-color, 10%), 5%);
// You may remove any of the following modules that you
// do not use in order to create a smaller css file.