Move Dashboard theme definitions to the components package

Make the `tkn--theme-*` classes available to consumers via the
common styles in the `@tektoncd/dashboard-components` package.

This means consumers do not need to define the themes themselves
unless they want to override the defaults.

packages/components/src/scss/_common.scss

@@ -11,8 +11,27 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+@use '@carbon/react/scss/themes';
 @use '@carbon/react/scss/theme' as *;
 
+:root, .tkn--theme-light {
+ @include theme(themes.$g10);
+}
+
+@mixin tkn--theme-dark {
+ @include theme(themes.$g90);
+}
+
+.tkn--theme-dark {
+ @include tkn--theme-dark;
+}
+
+@media (prefers-color-scheme: dark) {
+ .tkn--theme-system {
+ @include tkn--theme-dark;
+ }
+}
+
 .tkn--tooltip-trigger {
  appearance: none;
  padding: 0;

src/scss/_carbon.scss

@@ -79,24 +79,6 @@ limitations under the License.
  --tkn-button-border-width: #{button.$button-border-width};
 }
 
-:root, .tkn--theme-light {
- @include theme.theme(themes.$g10);
-}
-
-@mixin tkn--theme-dark {
- @include theme.theme(themes.$g90);
-}
-
-.tkn--theme-dark {
- @include tkn--theme-dark;
-}
-
-@media (prefers-color-scheme: dark) {
- .tkn--theme-system {
- @include tkn--theme-dark;
- }
-}
-
 .#{config.$prefix}--actionable-notification,
 .#{config.$prefix}--actionable-notification--toast,
 .#{config.$prefix}--inline-notification {