Корка светлая, оттого что духовка была холодной.

In Russian, the verb to be is normally omitted in the present tense.

So:

• Корка светлая = The crust is light/pale
• literally: Crust light

But in the past tense, Russian does use быть:

• духовка была холодной = the oven was cold

So the sentence naturally has no is in the first part, but it does have was in the second part.

Because it agrees with корка, which is a feminine singular noun.

Agreement here works like this:

• корка — feminine singular
• светлая — feminine singular form of светлый

If the noun were masculine, neuter, or plural, the adjective would change too:

• хлеб светлый — the bread is light
• тесто светлое — the dough is light
• корки светлые — the crusts are light

Here светлая means light-colored or pale, not bright in the sense of shining.

With food, светлый / светлая often describes color:

• светлая корка = a pale crust
• светлое пиво = light beer
• светлый хлеб = light-colored bread

So in this sentence it refers to the crust not browning enough.

Оттого что means because, since, or for the reason that.

In many cases, it is very close in meaning to потому что:

• Корка светлая, потому что духовка была холодной.
• Корка светлая, оттого что духовка была холодной.

Both mean basically the same thing.

A useful nuance:

• потому что is the most common everyday because
• оттого что can sound a little more literary or slightly more like as a result of the fact that

So learners should understand оттого что as a perfectly normal causal connector, just a bit less plain than потому что.

Because оттого что introduces a subordinate clause of cause.

Russian normally separates that kind of clause with a comma:

• Корка светлая, оттого что духовка была холодной.

The first part is the main statement:

• Корка светлая

The second part gives the reason:

• оттого что духовка была холодной

So the comma works much like English punctuation before a clause like because the oven was cold.

Because the subject is духовка, and духовка is a feminine singular noun.

In the past tense, Russian verbs agree in gender and number:

• он был — he/it was
• она была — she/it was
• оно было — it was
• они были — they were

So:

• духовка была = the oven was

Here холодной is in the instrumental case, and that is very common after быть in the past tense when an adjective is used as the predicate.

So:

• духовка была холодной = the oven was cold

This instrumental predicate is very natural in Russian. A learner will often see this pattern:

• Он был уставшим — He was tired
• Комната была тёмной — The room was dark
• Духовка была холодной — The oven was cold

You may sometimes encounter nominative predicate adjectives in other contexts, but была холодной is a standard and natural phrasing here.

Yes, literally it means cold, and it is natural here.

When Russians say an oven was холодная / холодной, they often mean:

• it was not heated
• it was not hot enough
• it was not preheated properly

So even if English might sometimes prefer the oven wasn’t hot enough or the oven was cold, the Russian wording is completely normal.

Because Russian is describing a current visible result and its past cause.

• Корка светлая — the crust is pale now / as we see it
• духовка была холодной — the reason is that the oven was cold earlier

This is very natural. English does the same kind of thing:

• The crust is pale because the oven was cold.

So the time contrast is not strange; it reflects present result + past explanation.

Yes. Russian often allows a more explicit wording if you want to stress the result of the baking process. For example:

• Корка получилась светлой, потому что духовка была холодной.

That means something like:

• The crust turned out pale because the oven was cold.

Compared with the original:

• Корка светлая, оттого что духовка была холодной.

the version with получилась sounds a bit more explicit about the outcome. The original is still correct and natural, especially if someone is simply pointing out the reason for the pale crust.