Blog Excerpt Add Read More

I have a theme that when I chose to only display a blog excerpt, (on my main blog page) it does not have a “Read More” at the end of the excerpt. I KNOW THAT I CAN A “READ MORE” text via WordPress, but I have to manually enter that, I don’t want that, I have adjusted my CSS so that the excerpt cuts off after 120 words (So the text ends at the same height as my images next to text). How and where would I enter CSS to add text that says “Read More”

Here is the CSS that I have used to cut off excerpt at 120 words.

//* Modify the length of post excerpts
add_filter( ‘excerpt_length’, ‘sp_excerpt_length’ );
function sp_excerpt_length( $length ) {
return 120; // pull first 120 words
}