Is there an antonym for "dim"/ synonym for "lighten?"

You could say the candle caused the room to gleam:

To emit a gleam; flash or glow

with the noun gleam defined as:

A steady but subdued shining; a glow

You could also use the verb kindle:

To cause to glow; light up: The sunset kindled the skies.

These both seem to convey the slight rise in light and not complete illumination you desire.

“Soften/ed,” when used with the notions of light and visibility, is usually associated with

“[mak[ing] (something) [in the usual case, the brighter/whiter extreme of visibility]) less severe, harsh, extreme, etc.].”
(i.e., the light of the room [was] softened = the room became less bright or darker)

However, the “(something)” being rendered “less severe, harsh, extreme, etc” in M-W’s definition could also be ...

“darkness,” the other extreme of visibility.
(i.e., the darkness of the room [was] softened = the room became less dark or brighter)

Three nice (imo) examples of this use of “softened” with “darkness” (to mean less dark or brighter) found in “Google Books” include (with emphasis added):

The darkness softened as dawn came sifting through the canopy of trees.”
(from ‘The Changeling Garden’ by Winifred Elze);

“The darkness softened as my eyes adjusted to it, … .”
(from ‘Alchemystic’by Anton Strout); and

“The candle on the desk went out, and suddenly the room was engulfed in shadow, the darkness softened only by firelight.”
(from ‘Lucy's Christmas Angel’ by Sandra Heath)