Issue number: internal
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## What is the new behavior?
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this PR. -->
- Added new common header partial, to be used by ionic theme and ios/md
- Renamed header.scss to header.native.scss. This inherits the common
created above and its used only on ios and md partial
- Created new partial for ionic theme.
- Added ionic theme to basic e2e tests.
## Does this introduce a breaking change?
- [ ] Yes
- [x] No
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## Other information
<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->
[Sample Header
Screen](https://ionic-framework-rcfex2fep-ionic1.vercel.app/src/components/header/test/basic?ionic:theme=ionic)
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Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Maria Hutt <maria@ionic.io>
