The Content Builder has a default Button content block. Without going to HTML I'm able to set a font, font size and colour. But how I make text bold?
This is not supported in the Content tab of the button block. However, if you're familiar with HTML, you can go to the HTML tab and add "font-weight: bold;" to the styles in A tag in the HTML.
Alternatively, since you have the font set to Arial, you might simply switch to Arial Black, which is basically just a bolder version of Arial.