So if you just installed the latest RubyMine for 2018 and installed the Material UI plugin, the whole RubyMine interface will use the Material design for the UI which means, everything is flat. If you don’t like it and prefer the old them, just uninstall the Material UI plugin.
Go to Settings then find the Plugins section. Search for Material UI and uninstall it. After a restart, you should get the glorious Dracula theme!