[TYPO3-english] RTE styles css on frontend

[Menelaos]

(Sorry if it is a naive question but I am a basic user of typo3.)

The styles applied to a text in RTE are not applied at frontend, because the class definitions are missing from css, (or class definitions are not substituted by inline css, id do not know how it is supposed to work)

For example if I make text align justify, the frontend html shows <p class="align-justify"> but the class is missing from the default classes (typo3tem/stylesheet...)

What I am doing wrong?

Do I have to add something in the TS config, or define a css with all RTE classes myself?