I'm studying japanese, and found this phrase:
I was thought that the particle that is always with 好き is が but in this case is は, is there an explanation for this???
Japanese grammar works this way.
In a clause, you can simply use case particles such as がのをにへ as it is.
Now, what if you convert the contents of the clause into a sentence? In English, you can put it as it is, i.e. "I like reading". However, it doesn't really work in Japanese. In other words, 私が読むのが好きだ can't immediately be a valid sentence. (It often happens to beginners that they carelessly produce topicless sentences or ones whose contents don't get along with an inferred topic.)
A sentence of statement needs some topics or equivalents in Japanese except some cases. Among several ways to produce topic parts, adding は to a noun phrase is one. Let's call this process "topicalization" here.
I'll omit the detail but anyway, when you topicalize 私が, it turns into 私は, and the sentence becomes 私は読むのが好きだ. Likewise, when you do 読むのが too, it produces 読むのは, thus, 私は読むのは好き.
If you are curious about the reason for using は*, searching difference between は and が will help. As user31035 says, expressing contrast is one of its functions (though it doesn't necessarily mean he hates the other things).
*; Expressing contrast is unique to は while so called thematic usage is a problem why you choose it rather than other choices especially in colloquial Japanese. But you can ignore the trivia for the time being.
I think it shows a contrast https://jisho.org/search/は. After reading this https://www.wasabi-jpn.com/japanese-grammar/ha-vs-ga-five-points-you-need-to-know/ it seems to me that the writer want to say that he/she only likes reading kanji and doesn’t like anything else about it.
But I am also new to Japanese. Let’s wait for someone more knowledgeable.
After reading user4092’s answer I see that my answer is wrong. I am leaving it here only because of the links as I think they are a good source of knowledge.