Condensation will sometimes collect on the inside of Honda Civic headlight assemblies. These units are built as sealed units; moisture inside them suggests there is a crack in the housing or the rear gasket, where the bulb is installed, needs to be replaced. You can replace the entire assembly with a new one. You can also locate a used one, if you can locate one in good condition. With some work, you may be able to take it out and fix it.
Open the bonnet of your Honda Civic, and identify the headlight assembly that has the condensation inside of it. Depending on the year of your Civic and side of the headlight, you may need to move the power steering reservoir to gain access to the light assembly.
Squeeze the clips on both sides of the headlight assembly to release the headlight connectors. Use your fingers to loosen the headlight, and pull the small plastic tab to loosen the rubber gasket from the frame.
Unplug the wiring harness from the back of the bulb. Twist the bulb, to remove it from the headlamp assembly.
Locate, and remove, the screws that hold the headlight casing in place. Once the screws have been removed, gently guide the headlight casing from the location.
Inspect the assembly under a bright light, identifying any cracks that may be in the casing. If there is a crack, use a silicone sealant to seal the crack.
Dry the assembly, using a towel and or hairdryer.
Replace the assembly back into your Honda Civic, and secure it by replacing the screws you removed.
Replace the rear gasket where the bulb is inserted with a new gasket. Insert the bulb, and twist it into place.
Reattach the bulb wiring harness.
Turn the lights on, and re-aim the light assembly. Inspect the assembly over the next few days, to see if the moisture returns.