* refactor(WIP): add new SCSS mixin `respond`
Theme's stylesheet mixes the use of max-width and min-width for responsive design, and it's horrible.
* refactor: update variable.scss to use @mixin respond
* refactor(grid): adjust sidebar max-width
* refactor(partials/article.scss): use `@include respond`
* refactor(layout/article.scss): delete unused SCSS and migrate to `@include respond`
* Adjust left sidebar max-width on xl screen
* fix: right sidebar not sticky
* refactor(breakpoint): simplify `@mixin respond`
* refactor(breakpoints): improve warning message
* fix(breakpoint): return only the requested breakpoint
* fix: add missing padding to .article-details
fix warning: found no layout file for "HTML" for kind "section": You should create a template file which matches Hugo Layouts Lookup Rules for this combination.
