[yetanother12345]

In your browser of choice, install the "Stylus" plugin. This is a plugin that will let you write custom CSS styles for any and all page(s). If your browser of choice does not have the exact "Stylus" plugin it will have a plugin of another (similar) name that will do the identical task.

As it seems from your post that you may not be extremely familiar with CSS, here is a ruleset that will do something close to what you wish. Font is set to 26px, not 14. You can easily change that.

Make it valid for "Everything" and it will be valid for everything but those sites that are extremely convoluted.

    *, html, body, section, article, div, span, p, i, b, strong {
        font-family: "Libre Sans" arial, helvetica, sans, sans-serif !important;
        font-size: 26px !important;
        font-weight: bold;
        line-height: 1.5em !important;
        background-color: white;
        color: black;
    }
    pre, code {
        font-family: "Libre Mono", Courier, monotype !important;
    }
   a {
        text-decoration: underline;
   }

[yetanother12345]

Added: Here is the ruleset I use for Hacker News specifically

    * {
        font-size: 23px;
        line-height: 1.5em
    }
    a {
        text-decoration: underline;
    }