The correct way to add your paragraph in the HTML block would be like this:
<p style="font-family:'Rock Salt';font-size:49px;color:#F8B509;text-align:center;">Explore Your Truth</p>
But you don’t need to use the HTML block, as the standard paragraph block has options to change the font size, color, and also to align the text centered.
The theme does not at the moment have the option to change the font family on a block-by-block basis, but you could use a plugin such as the Google Fonts Typography plugin that puts font family selection into the page editor.