Update the blog post about theming (after last HTML structure and styles adjustments)

[olayway]

Inline Tailwind classes has been replaced with single BEM-like semantic classnames (one per HTML element), and the HTML structure itself has been changed slightly too. Our blog post on customizing styles should be updated to explain the new approach to styling your Flowershow site (and mention that you can use official themes collection, and submit your theme there too for others to use).