Well, if you don't mind making a pact with some powerful entity, you could get 2 levels of warlock. This will give you access to a few spells, some of which perhaps being interesting (both stats and flavor-wise) for your character. But most of all, this will give you access to 2 invocations.
One of these invocations should be Devil's Sight, which grants you the ability to see through any form of darkness, magical or otherwise, for up to 120 feet. This is more than enough to cover the 15 feet of the darkness spell. Cast it on your clothes, and bring terror to your enemies.
While this is a good strategy (generally favored by blade pact warlocks), it has some limitations. While under the effect of the darkness spell, your allies cannot see you. As such, any spell requiring line of sight that they would like to cast on you simply cannot work. Healing you might be problematic, as it requires your allies to be able to touch you. They might know that you are smack in the middle of that sphere of pure black darkness but, once they get inside, it's suddenly not so easy to find you anymore.
These limitations also affect your enemies, however. Any attack from the outside of the sphere should have disadvantage as your enemies, even if they can approximate your position, can't exactly see you to aim properly. Enemies inside the darkness should also suffer from disadvantage when attacking you, unless they possess some form of blindsight or a similar ability. When you attack them, you should have advantage on the roll since you are functionally invisible for them.
The biggest cost of this strategy is the fact that it requires multi-classing. While the lvl 19 and 20 abilities for monk would not really be missed (realistically, few campaigns will reach these levels), it will still set you back 2 levels in obtaining you other core monk powers. Note that doing this would get you 2 invocations, however, and that some of them can be quite interesting for a ninja-esque character:
Armor of Shadows (cast mage armor at will, which might be better than
using your wisdom for AC if its under 16),
Eldritch Sight (cast detect magic at will),
- Eyes of the Runekeeper (allows you to read anything, even languages
you don't know... useful for spying!),
- Gaze of Two Minds (share the eyes of a willing target, more spying!),
- Mask of Many Faces (disguise self at will!), etc...
TL;DR
Sacrifice 2 monk levels and multiclass as a warlock. Gain some minor but potentially useful spellcasting, as well as the ability to see through magical darkness and one more ability of your choice!
In uniform, shared light levels, vision penalties due to darkness work fine, intuitively, and exactly as written (no surprises). The deeper the darkness, the more trouble you have seeing, hitting things, and protecting yourself.
This, however, isn't the case you're asking about.
Mixed lighting conditions
Let's stick with RAW to analyse this situation of two people in different lighting conditions. The results are almost entirely the same whether whether Aspartame is in magical darkness or the (possibly very dark) shadow of an oak tree. The results are going to be weird.
These results are pretty counter-intuitive. In real life, the rogue sneaking through the shadows has an advantage on everyone else, and you've probably handled it as such — the rogue themselves isn't blinded by being in the shadows, but they are in Pathfinder!
Pathfinder's vision penalties are counter-intuitive and weird because they don't correctly model vision as we know it: they give you trouble depending on the light level you're standing in, instead of giving you trouble based on the light level of the thing you're looking at. In reality I don't have trouble reading things in the dark because I'm in the dark, I have trouble because those things are.
Like I said though, they work fine when people are sharing the same light levels. They just weren't written with differing lighting conditions in mind and don't handle them well. The sacred RAW should be burned in this case upon the altar of This Could Have Been Written Better For These Cases. Rules are worth the results they produce, and here the results they produce are kinda dumb. Those who wish to apply it exactly because those are the rules can go ahead and suffer the headaches.
Drawing sense out of the ashes.
Pathfinder should probably just be modelling how things really work: darkness affects you based on what you're looking at, not what light level you're in. Accordingly, I'm making a recommendation that in mixed light levels, the RAW be interpreted for what it was probably intended to do — which just requires a small adjustment of the original RAW:
- Dim light: Being in dim light doesn't make you have trouble seeing, but looking at things that are in dim light means you have trouble seeing those things. Past that though the RAW works fine: things in dim light have concealment and you have a 20% miss chance against them.
- Darkness: Being in darkness doesn't make you blind or suffer penalties. However, you are considered blinded for the purposes of observing or interacting with anything that is in the dark. You have a 50% miss chance against those things in combat, you have penalties to AC and perception versus things in darkness, and so on.
- Alternately, stick with the Darkness spell's function, and just grant things in darkness total concealment and forget about blindness.
Wyrmwood brings up in comments that due to firing through concealment, firing past/through dim light introduces a 20% miss chance. This causes sense-making problems if you're on a long archery range, and a tall tree off to the side is casting a shadow across part of the middle of the range: those rules assert your target dummy now has concealment, despite still being in the bright daylight. These rules work fine for actual obstructions like black smoke, but not for light. I suggest that for the purposes of firing through concealment, don't count dim or dark squares — just count what it is you're targeting.
That produces these results, which are probably actually intuitive:
- While Aspartame is in dim light:
- Aspartame is considered as having concealment from Xylitol. Xylitol has trouble seeing him, and has a 20% miss chance.
- Aspartame fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- When Aspartame is in darkness:
- Xylitol is considered as being blinded for the purposes of trying to observe Aspartame. He has a 50% miss chance trying to attack Aspartame.
- Aspartame still fires without difficulty, because Xylitol is in bright daylight and clearly visible.
- Xylitol has all the defensive and sensory penalties for the purposes of any interaction between himself and Aspartame, because he can't see what Aspartame is doing at all.
We get these results, again, whether the dim light or darkness is due to magic or the shade of an oak tree.
Best Answer
How I usually see this ruled:
1) The Darkness spell causes the area in which it is placed to be limited to Dim Light, as you spell out clearly. It is, however, not a [light] effect and furthermore does not add to the lighting level, rather setting the cap on illumination in the area to Dim Light. This is usually equivalent to creating a region of pitch-black magical darkness in the situations where the spell is likely to be cast, because the spell also prevents lower level spell-based magical lighting from working at all so there's usually no viable source of light functioning in the area of effect. Mundane light sources, however, are merely limited to not being able to raise the light level above dim. Most adventurers don't bother carrying any mundane light sources past about level 2, however.
2) No, you'd need a light source for that.
RAW, as I understand it:
1) Yep!
2) Yep!