How to make font bold across the entire theme?

Greetings…are you wanting “everything” to be Arial bold? Do you have a link to your site as well?

Yes, everything would be great.
If I could control everything separately would be a plus but not a must.
the website is local in my computer.

You can apply the font to the body of your site to begin with like:

body,
.site-title,
h1,
h2,
h3,
h4,
h5,
h6,
.entry-title,
.entry-title a {
    font-family: arial,helvetica,sans-serif;
    font-weight: 600;
}

The CSS above will change everything in the body (most of your text in a page), the site title, and page/post headings to Arial. If Arial isn’t available, it goes to Helvetica, if that is not available, then it uses sans-serif.

Font weight is done as 600 (bold) but you can type in bold instead if you prefer.

Oh wow! It worked perfectly! Thank you so much for your help! … What about the main menu, how can I make those letters bold?

I’m glad it worked ??
As for the menu, just add this to the CSS:

.primary-navigation li

body,
.site-title,
h1,
h2,
h3,
h4,
h5,
h6,
.entry-title,
.entry-title a,
.primary-navigation li {
    font-family: arial,helvetica,sans-serif;
    font-weight: 600;
}

Oh man, thank you ever so much!