You can use Q2A Ultimate Widgets Plugin by QA-Themes (Towhid), that plugin has a feature which can turn your theme into dark mode. Basically, it turns all the colours to black and some borders and other important stuff to other colours so that you can get that dark mode feeling. But the matter of fact is, that it works and it's great ;)
Download from Github.
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If you have a question about Q2A, please ask here, in English.
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April 20, 2021: Q2A 1.8.6 released!