feat(theme): multiple values for nb-(except-)for-theme

[marchrius]

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• I read and followed the CONTRIBUTING.md guide.
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Short description of what this resolves:

Added mixins for both nb-for-theme and nb-except-for-theme to work with multiple theme names with same content style

[nnixaa]

Hey @marchrius, looks good to me. Could you describe your use cases for this feature?

[marchrius]

Hey @marchrius, looks good to me. Could you describe your use cases for this feature?

An example could be in the ngx-admin progect header.scss (see file reference)
https://github.com/akveo/ngx-admin/blob/d8e70922a92650a5490fdc7720ed115aeed55211/src/app/%40theme/components/header/header.component.scss#L62

Instead of duplicate the entire block to be used with multiple theme I can declare a list of themes that use the specific style declared once.

Eg.

  @include nb-for-theme(corporate) {
    $menu-action-separator-color: #3f4550;

    nb-action {
      @include nb-ltr(border-left-color, $menu-action-separator-color);
      @include nb-rtl(border-right-color, $menu-action-separator-color);
    }

    .header-container .logo {
      @include nb-ltr(border, none);
      @include nb-rtl(border, none);
    }

    .header-container /deep/ ngx-theme-switcher .dropdown-toggle {
      color: nb-theme(color-white);
      background: transparent;
    }
  }

  @include nb-for-theme(mytheme) {
    $menu-action-separator-color: #3f4550;

    nb-action {
      @include nb-ltr(border-left-color, $menu-action-separator-color);
      @include nb-rtl(border-right-color, $menu-action-separator-color);
    }

    .header-container .logo {
      @include nb-ltr(border, none);
      @include nb-rtl(border, none);
    }

    .header-container /deep/ ngx-theme-switcher .dropdown-toggle {
      color: nb-theme(color-white);
      background: transparent;
    }
  }

Can be replaced by

  @include nb-for-themes(corporate, mytheme) {
    $menu-action-separator-color: #3f4550;

    nb-action {
      @include nb-ltr(border-left-color, $menu-action-separator-color);
      @include nb-rtl(border-right-color, $menu-action-separator-color);
    }

    .header-container .logo {
      @include nb-ltr(border, none);
      @include nb-rtl(border, none);
    }

    .header-container /deep/ ngx-theme-switcher .dropdown-toggle {
      color: nb-theme(color-white);
      background: transparent;
    }
  }