This year in July (July 2016), Microsoft will discontinue the free upgrade offer for Windows 7 SP1 and Windows 8.1 users. If you have not decided to upgrade to Windows 10 due to some hardware restrictions, then you can still make your Windows 8 PC to have the appearance of Windows 10 using some of the themes developed by Windows enthusiasts. But you cannot install just any theme in Windows because the themes not digitally signed by Microsoft cannot be installed in Windows. To make a workaround, you have to replace some of the files inside Windows system and only then you can install unsigned third-party themes. Here is how:
First of all you have to replace some of the system files to be able to install third party themes. For this, you can use UltraUXThemePatcher. You can download this tool from http://www.syssel.net/hoefs/software_uxtheme.php?lang=en. It works for both Windows 8 and 8.1 systems. After the files replacement, you have to reboot Windows to load the newly replaced files.
After rebooting Windows, your PC is ready for the third-party Windows themes. You can download the Windows 10 themes for Windows 8/8.1 from the deviantart website at https://www.deviantart.com/sagorpirbd/art/Windows-10-Themes-for-Win-7-Final-489257538. You need a free DeviantArt account for downloading the theme package file. You have to unpack the contents of the Themes folder from the downloaded ZIP archive to the %WinDir%\Resources\Themes folder on your hard disk.
After copying all the theme files, you can go about using these newly installed themes the usual way. Right-click anywhere on the Windows desktop and choose Personalize from the context-menu. You will be able to see the new Windows 10 RTM themes. You can choose and apply any of these themes in your Windows 8/8.1 PC.
Even though the theme pack also contains Windows 10 wallpapers and Start button image resources, but it is not necessary to use them. If you want to use the the Start button resources, then you will need to install a third party start button like ClassicShell, Start8 or StartIsBack.