Update css quick guide approach

[david-ziffity]

Purpose of this pull request

This pull request (PR) adds a new section to the css quick guide approach about extending module styles.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/css-guide/css_quick_guide_approach.html

whatsnew
Added a new section about extending module styles to the Frontend Developer Guide.

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[jeff-matthews]

@david-ziffity, is this really that different from the preceding sections about extending and overriding component styles? They look almost identical.

[david-ziffity]

@jeff-matthews Yes it is different. The preceding sections are explaining about overriding or extending the Magneto's default ui component's style. The new one is explaining about overriding a module's styles, Since the module.less file is specific to modules and the file structure is different. But the overriding and extending concepts are same.

[jeff-matthews]

Thanks for clarifying.

[jeff-matthews]

Please see editorial suggestions.

[jeff-matthews]

Thanks @david-ziffity

[jeff-matthews]

running tests

[jeff-matthews]

@david-ziffity, tests failed due to markdown linting errors:

+ rake test:md
�[35mTesting Markdown style with mdl ...�[0m
�[31mThe Markdown linter detected 3 issue(s)�[0m
�[33msrc/guides/v2.3/frontend-dev-guide/css-guide/css_quick_guide_approach.md:99: MD012 Multiple consecutive blank lines
src/guides/v2.3/frontend-dev-guide/css-guide/css_quick_guide_approach.md:118: MD012 Multiple consecutive blank lines
src/guides/v2.3/frontend-dev-guide/css-guide/css_quick_guide_approach.md:141: MD012 Multiple consecutive blank lines

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

Please fix the errors.

[david-ziffity]

@jeff-matthews Fixed the errors. Please review it. Thanks for your support.

[jeff-matthews]

running tests

[ghost]

Hi @david-ziffity, thank you for your contribution!
Please, complete Contribution Survey, it will take less than a minute.
Your feedback will help us to improve contribution process.