SMPlayer

Open a video, then select the option 'Find subtitles on opensubtitles.org' within the Subtitles menu. A new window will show a list of suitable subtitles for the video you're playing. Select one and click on the Download button.

With some cards or video drivers, the hardware video equalizer may not work. Go to Preferences -> General -> Video and enable the software equalizer.

Go to the SMPlayer preferences, select the Interface section, then select the 'Skinnable GUI'. Now you can choose among several skins. In Linux, you need to install the package smplayer-skins.

The easiest way to find the configuration files of SMPlayer is by selecting the option 'Open configuration folder' in the Help menu. If you want to delete the current configuration and start with the default settings, just delete the file smplayer.ini (important: be sure SMPlayer is not running when you delete the file).

Go to View -> OSD -> Subtitles only.

Multithreaded decoding requires a recent build of MPlayer or FFmpeg-mt in Linux. MPlayer2 automatically uses all cores, but MPlayer requires manual adjustment. You can adjust the number of threads used for decoding in Preferences -> Performance. Set it to equal or less than the number of cores (and threads in the case of hyperthreaded Intel CPUs) your processor has.

You can play YouTube videos at 1080p or even at 4K resolution with SMPlayer. In Preferences -> Network, select mpv + youtube-dl in the option Support for video sites. Also, be sure mpv is selected as multimedia engine in Preferences -> General. This modification also makes the YouTube subtitles available in SMPlayer. Note: Linux users must install youtube-dl.