Free Drupal 8 themes overview
Drupal 8 already has list of modules that allow you to stamp little companies sites.
Free Drupal 8 themes are useful if you have an experience in layout coding. Just a few steps is a distance to a good company site on Drupal 8.
You can find lot of free contributed themes on drupal.org, but at this moment not all are ready to use. Archive names in the bottom of themes pages may have suffixes dev, alpha, beta. So this is not that will work surely. Dev - this is "development", the code may have errors. Alpha and Beta - these are versions that are stable, without errors, but not finished. To find themes that will work fine read this post.
Bartik - adaptive theme from Drupal core. It has 53 css-files, so it will be so hard to edit this theme and to apply your layout. Use another from the list below.
It's desirable to rename contributed theme that the updates will not erase your design.
The process of renaming Drupal 8 theme is described on Themy page.
Optimized Drupal 8 theme Themy
Optimal site's work, small page size, fast pages loading are available in clone of Paxton - adaptive theme Themy - it has no JS (but only if you have other modules with js), without unnecessary twig templates files, it has cascade menu with CSS (fast working).
Free ready to use Drupal 8 themes
BusinessGroup Zymphonies Theme. Pretty decent theme for a business website, adaptive, as well as all Zymphonies themes.
Paxton. Ready to use theme for Drupal 8. Quality beautiful design, simple adaptability, beautiful parallax slider with the foreground and background. Suitable for a small blog with big pictures.
Professional Responsive Theme. Ready for Drupal 8. Has good adaptability to the small screens, squeezing Main Menu to a button on the small screens. Although fonts and colors are difficult to read, but the theme is very high quality. Suffice it to edit styles. Demo
Business Responsive Theme. Nice theme by Zymphonies. Just a few improvements and color edition will give you good company site.
Bootstrap Rainbow. Ready. Simple theme for the site-portfolio. Adaptive, customizable columns 1,2 or 3, nice fonts.
Nero. Colorful menu is based on Bootstrap, adaptive. Positioning of the Main Menu is not debugged.
Jethro. Adaptive theme for portfolio and small website. Colorful blue and yellow theme with a gray background and uncomfortable fonts. Based on Bootstrap. 1, 2 or 3 columns.
OpenChurch Theme. Ready. For magazine, blog.
Base themes for Drupal 8
FortyTwo. Ready. Basic white theme to create their own sub-themes. Adaptability, a few settings in the admin panel: the location of the columns, enable / disable JS, separator breadcrumbs.
Bootstrap. beta3 only. Powerful theme for experienced developers that integrates Bootstrap framework http://getbootstrap.com/. To create own theme based on Bootstrap see the documentation http://drupal-bootstrap.org
Beginning. beta3 only. White theme for exercising responsive layout from scratch. Cross-browser (IE>9, Firefox, Chrome, Opera, Safari), HTML5.
Omega. Alpha5 only. Powerful basic theme popular among drupalers. Originally white, is used to create subthemes.
AdaptiveTheme. Only dev. One of the very productive, adaptive. Originally white, with no elements. It is convenient to create sub-topics. It has the most powerful admin panel.
Basic. Cannot enable because of the error: "The website encountered an unexpected error. Please try again later." Adaptive theme popular among Drupal-specialists. SASS, seo-optimized, column system, rich admin panel, user-friendly design to develop.
On the development
Personal Blog Theme. Only Alpha1. Multipurpose exercising responsive theme, black background, promotional area, right sidebar. Compatible with IE, Firefox, Chrome, Safari. HTML5, Css3, SASS Botstrat, image gallery.
ZURB Foundation. Only beta2. Grid system.
Ombak. nly maids. Exercising responsive template. Promo region. Many regions. Multipurpose.
Insha. Only dev. White blog theme.
How to install Drupal 8 theme
Enable a module **Update Manager** and go to "Appearance", i.e: /admin/appearance. Click on "Install new theme" and input archive file url, that you can find in the bottom of each theme page. If files will not download automatically extract theme folder to the folder **themes** at the root of drupal. Then a new theme will have appeared on page "Appearance". Press the link "Set as default" and use it.
In Drupal 8 contributed (user's) themes must be placed in a folder "themes", but not in sites/all/themes as it was in Drupal 7.