Mit dem Textmarker markiere ich nur die wichtigsten Sätze, sonst wird die Seite zu bunt.

Because mit always takes the dative case in German.

• der Textmarker = nominative
• dem Textmarker = dative

So mit dem Textmarker means with the highlighter.

Also, Textmarker is a masculine noun: der Textmarker.

Textmarker means highlighter, the pen used to mark text with bright color.

It is made from:

• Text = text
• Marker = marker

So mit dem Textmarker is naturally with the highlighter.

In a normal German main clause, the finite verb must be in second position.

Here, the sentence begins with Mit dem Textmarker, which counts as the first element. That means the verb must come next:

• Mit dem Textmarker | markiere | ich ...

If you started with ich, then you would say:

• Ich markiere mit dem Textmarker nur die wichtigsten Sätze.

Both are correct. The version in your sentence emphasizes with the highlighter first.

It is accusative plural, because it is the direct object of markieren.

The verb markieren needs something that is being marked:

• Ich markiere was?
  die wichtigsten Sätze

So:

• die Sätze = the sentences
• die wichtigsten Sätze = the most important sentences

Because the adjective must agree with the noun in number, case, and article pattern.

Here you have:

• die = plural definite article
• Sätze = plural noun
• adjective in the superlative form = wichtigsten

After a definite article in plural accusative, the adjective takes -en:

• die wichtigsten Sätze

So wichtigsten is the correct form.

It is the superlative form of wichtig:

• wichtig = important
• wichtiger = more important
• am wichtigsten = most important

Before a noun, German often uses the attributive superlative form:

• der wichtigste Satz = the most important sentence
• die wichtigsten Sätze = the most important sentences

So here it means that only the sentences with the highest importance are being marked.

Nur means only.

In this sentence, it limits what the speaker marks:

• Ich markiere nur die wichtigsten Sätze
  = I only mark the most important sentences

The idea is: not everything, just the key sentences.

Because the sentence contains two main clauses:

1. Mit dem Textmarker markiere ich nur die wichtigsten Sätze
2. sonst wird die Seite zu bunt

In German, two independent clauses are usually separated by a comma.

Here sonst means something like:

• otherwise
• or else

So the comma helps divide the two complete thoughts.

Because German main clauses still follow the verb-second rule.

Here, sonst takes the first position, so the verb must come second:

• Sonst | wird | die Seite | zu bunt

This is exactly like:

• Heute gehe ich nach Hause.
• Dann kommt er.
• Sonst wird die Seite zu bunt.

Here sonst means otherwise.

It introduces the consequence if the speaker does not limit the highlighting:

• I only highlight the most important sentences, otherwise the page becomes too colorful.

So sonst connects the first clause with a warning or unwanted result.

Because werden means to become, while sein means to be.

• Die Seite ist bunt = the page is colorful
• Die Seite wird bunt = the page becomes colorful
• Die Seite wird zu bunt = the page becomes too colorful

Here the idea is a change of state: if you highlight too much, the page ends up becoming overly colorful.

Zu means too in the sense of excessively.

So:

• bunt = colorful
• zu bunt = too colorful / overly colorful

The speaker is saying that too much highlighting makes the page visually overloaded.

Yes, in context it can suggest more than just colorful.

Literally, bunt means colorful, but in a sentence like this it often carries the idea that the page looks:

• too bright
• too crowded with color
• visually distracting

So zu bunt may feel a bit like too colorful or too visually busy.

Because the speaker is talking about the page as a general result in the situation at hand.

German often uses the singular when referring to the specific page being worked on, or to a page in a general practical sense:

• If I highlight too much, the page becomes too colorful.

It does not need to be plural unless you really mean several pages:

• ... sonst werden die Seiten zu bunt.

Yes. Both are possible, but they mean slightly different things.

• mit dem Textmarker = with the highlighter
  This often refers to a specific highlighter, or the highlighter as the tool being used in the situation.
• mit einem Textmarker = with a highlighter
  This is more indefinite: any highlighter, one highlighter.

So both are grammatically fine, but dem sounds more tied to a specific context.

Yes, markieren is very common and natural.

It can mean:

• to mark
• to highlight
• to indicate

In the context of a Textmarker, it clearly means to highlight.

You could also hear verbs like anstreichen in some contexts, but mit dem Textmarker markieren is perfectly normal German.

Yes. German allows different word orders as long as the verb-second rule is respected in the main clause.

For example:

• Ich markiere mit dem Textmarker nur die wichtigsten Sätze, sonst wird die Seite zu bunt.
• Nur die wichtigsten Sätze markiere ich mit dem Textmarker, sonst wird die Seite zu bunt.

These versions shift the emphasis:

• starting with Ich emphasizes the speaker
• starting with Mit dem Textmarker emphasizes the tool
• starting with Nur die wichtigsten Sätze emphasizes the limitation

The original sentence is perfectly natural.