The Underdeep karma good and evil path starts at Neutral (+0) and moves toward Good (+1) or Evil depending on how your character behaves in the dungeon - and in the current build karma gives you nothing mechanical at all. It is a roleplaying axis and a route decision, not a build decision.
Underdeep karma good and evil path: how it moves with your behaviour
Karma is shown on the character panel directly under your name and job, starting at Neutral (+0) and moving up to Good (+1) as you behave well. It shifts with how your character acts inside the dungeon.
F and G markers decide what your character does
The two behaviour policies are the direct lever: F is Do Evil and G is Do good, and you can place them on individual tiles so a route decides your character's behaviour as it passes. A tile marked F makes your character do something evil on the way through; G makes it do something good. Placing those markers is covered in the route settings guide.
Events let you spend karma
Karma also shows up in the events. Most of the one-time events on the map give you a choice between passing a skill check and taking a hit to your karma to skip it, so the light or dark path is something you can spend deliberately.
One practical note: because karma is spent on event shortcuts, taking the shortcut when a check would have passed burns a resource you may want to keep for a later event. The checks themselves are covered in the negotiation and Charisma events guide.
What karma does not do
In the current build, karma does not give you anything. There is no stat, drop, shop or unlock effect attached to good or evil karma - the value is the roleplaying and the route it gives you, not a mechanical reward. That also means there is no penalty for staying neutral, so a route that uses F or G is a flavour choice rather than a build decision.