Тематический раунд оказался сложным.

Because it agrees with раунд, which is a masculine singular nominative noun.

• раунд = masculine
• singular = one round
• nominative = the subject of the sentence

So the adjective also has to be masculine singular nominative:

• тематический раунд

This is the basic dictionary-style agreement pattern:

• masculine: тематический
• feminine: тематическая
• neuter: тематическое
• plural: тематические

It is in the nominative case because it is the subject of the sentence.

In this sentence, раунд is the thing that turned out to be difficult. That makes it the grammatical subject, so nominative is used:

• Тематический раунд = the thematic round

Because after оказаться in this meaning, Russian normally uses the instrumental case for the predicate adjective.

So:

• раунд оказался сложным
• literally: the round turned out difficult

This is a very common pattern:

• Он оказался правым. = He turned out to be right.
• Фильм оказался скучным. = The film turned out to be boring.
• Задача оказалась трудной. = The task turned out to be difficult.

Here, сложным is the masculine singular instrumental form of сложный.

Оказался is the past tense, masculine singular form of оказаться.

In this kind of sentence, оказаться means:

• to turn out to be
• to prove to be

So оказался сложным means:

• turned out to be difficult
• proved difficult

It often suggests that the result was discovered later, or was different from what someone expected.

Compare:

• Раунд был сложным. = The round was difficult.
  This is more neutral, just stating a fact.
• Раунд оказался сложным. = The round turned out to be difficult.
  This suggests discovery or realization.

In the past tense, Russian verbs agree with the subject in gender and number.

Since раунд is masculine singular, the verb is also masculine singular:

• masculine: оказался
• feminine: оказалась
• neuter: оказалось
• plural: оказались

Examples:

• Игра оказалась интересной. = The game turned out to be interesting.
  (игра is feminine)
• Задание оказалось лёгким. = The task turned out to be easy.
  (задание is neuter)
• Вопросы оказались трудными. = The questions turned out to be difficult.
  (вопросы is plural)

Not exactly.

Both are related to theme, but тематический is the more standard and common choice in many formal or neutral contexts, especially when something is organized around a theme or topic.

So тематический раунд is a very natural phrase for:

• a themed round
• a topic-based round
• a thematic round

Тематичный exists too, but it often feels more like:

• fitting the theme well
• thematically appropriate
• strongly themed in style or appearance

So in this sentence, тематический is the safer and more natural word.

Yes. Russian word order is flexible, although the most neutral order here is:

• Тематический раунд оказался сложным.

You could also say:

• Раунд оказался сложным.
• Сложным оказался тематический раунд.

The meaning stays basically the same, but the emphasis changes.

For example:

• Сложным оказался тематический раунд puts more focus on сложным
• Тематический раунд оказался сложным is the most straightforward, neutral version

Russian often uses word order to show emphasis rather than changing the basic grammar.

Yes, absolutely.

That would also be correct:

• Тематический раунд был сложным.

The difference is subtle but important:

• был сложным = simply was difficult
• оказался сложным = turned out to be difficult

So оказался adds a sense of result, discovery, or surprise.

If you just want to describe the round, был is fine. If you want to say that people found out it was difficult, оказался is better.

Сложным is the instrumental masculine/neuter singular form of сложный.

Here is the adjective pattern:

• nominative masculine: сложный
• nominative feminine: сложная
• nominative neuter: сложное
• nominative plural: сложные

Instrumental singular:

• masculine/neuter: сложным
• feminine: сложной

So the sentence uses instrumental because of оказался:

• раунд оказался сложным

Yes. Раунд is a borrowed word, originally from English, but it is fully used in Russian.

It is common in contexts like:

• games
• quizzes
• sports
• competitions
• discussions with stages or rounds

Even though it is a borrowing, it behaves like a normal Russian masculine noun:

• раунд
• раунда
• раунду
• раундом, etc.

So grammatically there is nothing unusual about it in this sentence.

Everything would change to match the new subject, and the final adjective would still usually stay in the instrumental after оказаться.

For example, with a feminine noun:

• Тема оказалась сложной.
  = The topic turned out to be difficult.

With a neuter noun:

• Задание оказалось сложным.
  = The task turned out to be difficult.

With a plural noun:

• Вопросы оказались сложными.
  = The questions turned out to be difficult.

So two things agree with the subject:

1. the past tense verb
2. the adjective after оказаться

A helpful approximation is:

• tye-ma-TI-che-skiy round a-ka-ZAL-sya SLOZH-nym

A few useful pronunciation notes:

• тематический has stress on ти: те-ма-ТИ-че-ский
• оказался has stress on зал: о-ка-ЗАЛ-ся
• сложным has stress on слож: СЛОЖ-ным

Also:

• ч in Russian is always like ch in chop
• ж sounds like the s in measure
• ы in -ным is a Russian vowel with no exact English equivalent

The pattern is:

• [subject in nominative] + [оказаться in past tense] + [predicate adjective in instrumental]

So here:

• Тематический раунд = subject
• оказался = past tense verb
• сложным = predicate adjective in instrumental

This is a very useful Russian sentence model. You can reuse it with many words:

• Экзамен оказался трудным. = The exam turned out to be difficult.
• Фильм оказался интересным. = The film turned out to be interesting.
• Решение оказалось правильным. = The solution turned out to be correct.