Quite simply, no, I don’t think any such thing exists in official publications. Quite simply, I think, it’s too small to bother writing up—DMs are supposed to be modifying and adjusting and making the published materials work for their game. To print something like this would be, in my speculation, be considered a waste of Wizards of the Coast’s extremely valuable and limited page space—their position would be that a DM doesn’t need something like this spelled out for them.
On top of that, there’s the issue that you want something that shines through darkness—that is simultaneously weaker than light. That’s almost a contradiction in terms, and almost-certainly doesn’t exist.
Nonetheless, D&D 3.5e is absolutely massive, so even with all those reasons for this to not exist, something fairly similar actually does.
Circle of the third eye warforged component
It’s not a template, and not something you can apply to just any character, but the circlet of the third eye attaches to a warforged’s forehead, and, well, gives them a third eye, on an eyestalk. The reason this is relevant is because
The eye glows a bright yellow and is clearly visible even in darkness or deep shadow (all Hide checks are made at a −5 penalty).
That gives you mechanics you could use for this, though of course you’d have to apply it to these creatures as a houserule, rather than only having warforged who all use this component.
Again, note that “darkness” here isn’t darkness; the spell would presumably still overcome this light.
You could, of course, dim the lights a little bit, and say the Hide penalty only applies in darkness or deep shadow.
Other not-nearly-so-close options
Aside from that, everything remotely close I can come up with just reinforces the idea that this is just too minor a detail for Wizards of the Coast to publish: all of the effects are vastly more potent than you seek.
Saint template
Book of Exalted Deeds has this preposterously overpowered template, which includes this bit:
A saint looks no different than she did before achieving sanctification, though she is surrounded by a palpable aura of holiness that sometimes (though not always) takes visible form in her protective aura of light.
Presumably this refers to the saint’s protective aura,
a nimbus of light having a radius of 20 feet. This acts as a double-strength magic circle against evil and as a lesser globe of invulnerability, both as cast by a cleric whose level equal to the saint’s Hit Dice.
That, obviously, is extremely powerful and probably inappropriate for the purposes you describe, and it doesn’t actually offer anything about, ya know, light and sneaking and stuff. I guess they imagined that saints never have cause to be stealthy.
Phantom Sparks flaw
Dragon vol. 327 has the Phantom Spark flaw,
You spontaneously emit bursts of colored light. These lights are completely out of your control and cannot be used as a light source. They only cease when you are unconscious, petrified, asleep, or dead. You suffer a −4 penalty on all Hide and Spot checks.
As with all flaws, the creature with this flaw gains an extra bonus feat. These effects—minor though they are—are still more significant than you describe, because you don’t want their glowing to be enough to stand out in decent lighting, so the Hide penalty shouldn’t apply except in darkness, and the Spot penalty is too much regardless. It’s still the smallest thing I can find.
Radiant creature template
Dragon vol. 321 has the radiant creature template, which can be added to any aberration, animal, dragon, fey, giant, humanoid, magical beast, or monstrous humanoid.
Radiant creatures are natives of the Plane of Radiance and are at least partially made up of the raw materials and energies of that plane. Most radiant creatures are bright, luminous versions of beings found on the Material Plane. A radiant creature is the same size and shape as its Material Plane counterpart, but it appears as a semi-translucent mass of shimmering, shifting colors and sparkling lights.
Despite the claim that they are “semi-translucent,” the accompanying art of a radiant giant eagle does not appear to be any such thing.
Radiant creatures gain a rainbow aura special attack, which says that they
constantly shed a glaring aura that most creatures find difficult to look at. Those within 30 feet of a radiant creature must make a Will save (DC 10 + ½ the creature’s racial Hit Dice + the creature’s Charisma modifier) or be dazzled for 1d6 rounds, suffering a −1 penalty on attack rolls, Search checks, and Spot checks. Radiant creatures are immune to the rainbow aura of other radiant creatures.
They also gain color spray, blur, searing light, freedom of movement, rainbow pattern, prismatic spray, repulsion, scintillating pattern, and prismatic sphere as spell-like abilities, at least if they have enough HD, along with immunity to the blind, dazzled, dazed (!), and stunned conditions.
