Remove warnings for new Sass "Mixed Declarations"

[sebj54]

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• [x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [x] You have signed the CLA.
• [x] All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• [x] Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

Deprecation warnings, see #302

What is the new behavior?

No more deprecation warnings

Closes #302

I also updated sass because of a build error described here: https://github.com/NativeScript/theme/issues/293#issuecomment-1008366830

[NathanWalker]

Released with https://github.com/NativeScript/theme/tree/v3.1.1

[sebj54]

Thansk for the quick merge!