Slider doesn't look like in the preview - usually the font is different
- Font color/size is different, than in the preview/font manager.
Many people use CSS codes with !important property, or other strong css codes without specifying which part of the website should it affect, which is not a good thing, because it overwrites the proper codes. To find out what codes affect the parts of your slider check out this documentation.
Another problem could be, if you only have problem with the boldness of your font is, that if you are using Google fonts, and that Google font is called in by another code too, that could overwrite ours, because Google fonts have an exact name, so there can be only one font with that name in your website. This can be related to the boldness, because there are different boldnesses, that can be called in from Google fonts, and if this other plugin's/extension's or theme's/template's code only calls in a certain boldness, then that is the only font weight, how this font can show up! So the solution is to remove that other code's Google font incalling, or modify it to call in other font weights too.