Перевод - Французский-Английский - Dans les ténèbres, sois ta propre lumière.

I don't know whether it is important, but in the French version the stress was put on the darkness (gloom?) first and it does not appear in the English version. Would it be incorrect in English to type "In the darkness, be your own light" instead?

I think that the order of the words and the comma do give an added value to the text.

In my opinion, in this case preposition "dans" should be rendered as "under", for it does indicate a state of person. You may translate "dans" as "in" when it does indicate a place or an event. Depending on context you may choose one of the options as written below:

(a) Under the darkness be light for yourself.
(b) Surrounded by darkness ...
(c) In front of darkness ...