1. Changing theme of Spring Tool Suite without downloading any plugin.
(For using Darkest Dark Theme see option 2)
First, go to
Window > Preferences

Then in Preferences dialog go to
General > Appearance
after that select *Dark in the dropdown for Theme, after that click on Apply, then on Apply and Close

Then restart STS once so your STS will look like this

2. Changing theme of Spring Tool Suite by downloading Darkest Dark plugin.
First go to
Help > Eclipse Marketplace...

Then in Eclipse Marketplace dialog search for Darkest Dark and then click on Go.
After that from the search results install the plugin named Darkest Dark Theme with DevStyle

After that with default options selected click on Confirm

Then accept the license and click on Finish

Then let the software install

After installation completes click on Restart in the pop up

Then in the welcome screen click on Next

After that select Darkest Dark Theme and click on Next

Then let the STS launch with Darkest Dark Theme

Then click on Continue in the Tweak the Workbench dialog

Then your STS will look like this
