I would also like to change the color of the font under the posts where it says the dates and number of comments. Basically everything on my page that has that faded grey color I would like to change to black.

That color code is #aaa. To change everything that's using that color code to something else, open your theme's stylesheet which you can find by clicking "Edit" next to "Mode: Add-on" on your Appearance → Custom Design → CSS page.

Here is your current theme stylesheet link:
https://s1.wp.com/wp-content/themes/pub/oxygen/style.css?m=1360153398g&minify=false

Search for #aaa, and copy all of the selectors that have that set as the color (example: ".entry-summary .page-links") and make them into a comma-separated list and then add a CSS rule like this:

YOUR_LIST_OF_SELECTORS {
	color: #000;
}

Replace YOUR_LIST_OF_SELECTORS with the list you found when you searched the stylesheet.

One more thing, how would I change the font color of the widget title to black?

You were close! In your theme, the class name for widget titles is "widgettitle" not "widget-title" so you would want to set it up like this:

.widgettitle,
.widgettitle a {
	color: #000;
}

I'd like to change the font size for my primary menu but I am having no luck doing this in the CSS style editor.

I checked out your blog at http://dappermanstyle.wordpress.com/ and I see that it is currently using the Oxygen theme.

You can use the following CSS as a starting point for changing any of the colors associated with the main menu in the Oxygen theme by adjusting the color codes and adding the final CSS to your Appearance → Custom Design → CSS editor:

.main-navigation ul ul {
	background-color: #fff;
}
.main-navigation a,
.main-navigation > div > ul > li > a {
	color: #f00;
}
.main-navigation a:hover {
	color: #0ff;
}
.main-navigation li.current_page_item a,
.main-navigation li.current-menu-item a {
	color: #b00;
}
.main-navigation li.current_page_item li a:hover,
.main-navigation li.current-menu-item li a:hover,
.main-navigation li li.current_page_item a,
.main-navigation li li.current-menu-item a {
	color: #b00;
}

.widget-title a {
color: #000;
}

but it has not been working.

I would also like to change the color of the font under the posts where it says the dates and number of comments. Basically everything on my page that has that faded grey color I would like to change to black.

Thank you so much in advance.

Any suggestions on what code to enter?

Thanks!