Sections LESS mixins: fix the issue with missing rules and incorrect default variables

[dmytro-ch]

There are couple of issues when trying to use mixins for "tabs" and "accordion".

The issue is related to lib/web/css/source/lib/_sections.less.
The default variables are predefined in lib/web/css/source/lib/variables/_sections.less.

Description (*)

When you try to customize the view of "tabs" and "accordion" components by redefining the default variables in the scope of a custom theme, the following issues appear:

1. The text color for active "accordion" control is not applied.
2. The text-decoration for active control is not working for both "accordion" and "tabs" components.
3. There are also incorrectly defined default variables for mixins. (e.g. accordion control visited and tab control hover variables etc.).

Fixed Issues (if relevant)

1. Bug in "_sections.less" mixins: missing rules and incorrect default variables #18729: Bug in "_sections.less" mixins: missing rules and incorrect default variables

Manual testing scenarios (*)

Let's try to define the following variables in the scope of a custom theme.

@tab-control__active__background-color: yellow;
@tab-control__active__color: red;
@tab-control__active__text-decoration: underline;

Assuming that those variables are not defined elsewhere (in the scope of current or parent theme) except the default lib variables file lib/web/css/source/lib/variables/_sections.less.

Expected Result

There are red color, yellow background and underline text decoration applied for both accordion and tabs active controls.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3239 has been created to process this Pull Request

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3239 has been created to process this Pull Request

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3239 has been created to process this Pull Request

[magento-engcom-team]

Hi @dmytro-ch. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.