В кладовке темно, поэтому я ищу выключатель.

В кладовке means in the pantry / in the storage room (location, not motion).
With в:

• в + Prepositional answers Where? (Где?) → location: в кладовке
• в + Accusative answers Where to? (Куда?) → movement: в кладовку (into the pantry)

So В кладовке темно = It’s dark in the pantry.

Russian often omits the present-tense verb “to be” (есть) in ordinary statements.
So В кладовке темно literally is In the pantry [it is] dark.
You can add было/будет in past/future:

• В кладовке было темно. = It was dark.
• В кладовке будет темно. = It will be dark.

Темно here is a predicative adverb / category of state (often called a “state word”). It describes the overall situation/environment: It’s dark.
You’d use an adjective like тёмная only if you had a noun to describe:

• Тёмная кладовка = a dark pantry (adjective + noun) But when you mean “it’s dark (there),” Russian commonly uses темно.

They’re related but not interchangeable:

• потому что = because (gives the reason)
  Example: Я ищу выключатель, потому что в кладовке темно.
• поэтому = therefore/so (gives the result)
  Example: В кладовке темно, поэтому я ищу выключатель.

So потому что introduces the cause; поэтому introduces the consequence.

Я is optional because Russian verbs show the subject in the ending. You could say:

• … поэтому я ищу выключатель. (neutral, explicit “I”)
• … поэтому ищу выключатель. (also normal, “I” implied)

Word order is flexible and used for emphasis:

• … поэтому я ищу… emphasizes I (as opposed to someone else) a bit more.
• … поэтому ищу я… sounds more contrastive/poetic and is less neutral in everyday speech.

It’s the direct object of искать (to look for), so it takes accusative.
For many masculine inanimate nouns, accusative = nominative, so выключатель stays the same:

• nominative: выключатель
• accusative: выключатель

If it were masculine animate, accusative would usually match genitive, but выключатель is inanimate.

• выключатель = (light) switch (the physical device)
• выключить = to turn off (a verb)
• выключение = switching off / shutdown (an action/noun)

So here it specifically means the wall switch you press/flip.

Yes, but it changes the meaning:

• …ищу выключатель = I’m looking for the switch (to turn on the light)
• …ищу лампу = I’m looking for a lamp (a physical lamp)
• …ищу свет sounds odd literally (“looking for light”), but you can say things like ищу источник света = looking for a light source.

Темно generally means dark / not lit enough, ranging from very dim to pitch-black depending on context. If you want to stress “pitch-black,” Russian often uses:

• совсем темно = completely dark
• очень темно = very dark
• кромешная тьма = pitch darkness (more literary)

Yes. If you want to refer back to a previous statement, you can start with Поэтому:

• В кладовке темно. Поэтому я ищу выключатель. This splits it into two sentences and can feel slightly more emphatic.