The plugin, developed by +fossfreedom (who's also behind the Rhythmbox CoverArt Browser plugin) in collaboration with +Ozon and +Georgi Karavasilev, comes with options such as display type: use header bars or a regular/compact toolbar, optional compact progress bar and more:
From the header bar, you can switch between songs and categories as well as search/filter while the play controls are at the bottom, with customizable song info position, regular/compact progress bar, optional volume control and more.
Under desktop environments that don't fully support client-side decorations, the plugin is still useful because it allows you to enable a compact toolbar similar to the one available in Rhythmbox until version 2.99:
The toolbar can be hidden from the menu: View > Show Play-Controls Toolbar and you can then use Alt+Right to seek forward and Alt+Left to seek backward.
In his article about the Rhythmbox Alternative Toolbar plugin, fossfreedom asks for feedback and "most importantly help with further development" so if you can help, drop by the project page or leave your feedback in the comments.
For Ozon OS, Rhythmbox will get even more changes: the sidebar and coverart plugins will be redesigned and the Alternative Toolbar plugin will be further improved. You can watch THIS video for more info.
Install Rhythmbox Alternative Toolbar plugin
The Rhythmbox Alternative Toolbar plugin is available in fossfreedom's Rhythmox plugins PPA (along with many other third-party plugins), for Ubuntu 15.04, 14.10 and 14.04 / Linux Mint 17.1 or 17. To add the PPA and install the plugin, use the following commands:sudo add-apt-repository ppa:fossfreedom/rhythmbox-plugins
sudo apt-get update
sudo apt-get install rhythmbox-plugin-alternative-toolbarOnce installed, enable Alternative Toolbar via Tools > Plugins.
Arch Linux users can install Rhythmbox Alternative Toolbar plugin (git) via AUR. For other Linux distributions, see the installation instructions from the Rhythmbox Alternative Toolbar plugin GitHub page.