Light and dark tones now have no mechanical effect - there are no counters or direct effects on other cards. The descriptive / storytelling effect is the whole of the point.
(I almost wrote 'only a descriptive effect', but in Microscope there's nothing 'only' about description. Tone is very important; try playing without it and you'll see the difference pretty quickly. Tone generates discussions of expectations about the period, and thereby helps the group achieve a shared vision of the history.)
For the curious, here's the play-test history you're thinking of and why it was changed:
You're correct that the Tone rules are the only survivors from earlier versions in which they provided mechanical enforcement.
Originally, light and dark counters on the cards were used to track 'tone debt'. I suspect you're thinking of the v3 playtest, in which tone debt was tracked separately with light/dark tone counters on each card in the timeline.
The contribution of a parent to its children's tones worked by giving tone debt to children that didn't fit. For example, placing a dark Scene in a light Period would call for the placement of an additional light tone debt counter, to reflect that the overall tone should drift a little back towards light.
Tone debt could then be invoked to take control of scenes in additional ways. That light tone debt could be 'called in' by a player to take over an NPC or other player's PC, establish new facts that changed the scene, or 'predict' an enforceable outcome of events - as long as this made the outcome more light. Thus, scenes in children were given a tendency towards the tone of the parent.
In v4, these rules became simpler, more abstract and smoother in use. Tone debt (now renamed 'Drama') sat only on Periods, and could be used for any scene in the period. Crucially, however, the 'finger voting' rules were introduced. Initially these were a conflict-resolution system for use of Drama. Instead of giving you enforced control of an outcome or fact, spending a Drama let you propose a light/dark fact or outcome, to be resolved by finger-vote.
And those rules turned out to make the concept of tone debt/drama counters obsolete.
The useful gameplay and conflict resolution functions were being handled by the finger voting. The old ability to predict/enforce scene ends in advance simply became unnecessary, because there's now a way to guide scenes to that end with a Push.
And limiting a player's ability to introduce facts and control NPCs was no longer useful; instead of adding extra powers, Drama increasingly became a supply-of-counters limit on a something that players should be able to do all the time. The remaining value of tone was to force the players to discuss the meaning of the history - which is accomplished by the act of deciding the tone, and not aided by tone debt enforcement in scenes.
So the Drama rules were eventually transmuted into the Push rules, which have the same function but don't require you to put counters on the board first. (The Legacies were also heavily revised, as they tied in with that system.)
For more on these reasons, see the author's post looking back on the changes.