fix: Bootstrap and .nt-form colors not adapting to skin

[bundyo]

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/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.
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• [x] All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests.
• [ ] Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

Currently .text-primary and .nt-form__link classes are not dependent on skin colors, while they should be.

What is the new behavior?

.text-primary and .nt-form__link classes change colors with the current skin.

Fixes #245.