How do I add a bullet or dash to Widget links?

you will find this in style.css:

#side .widget li {clear: left;}

edit it and add a ‘list-style-type’:
(see https://www.w3schools.com/CSS/pr_list-style-type.asp for options, and also https://www.w3schools.com/CSS/css_list.asp)

#side .widget li {clear: left;
list-style-type: disc;
list-style-position:inside;
/*optional to try*/
text-indent:-12px;
padding-left:10px;
color:#bd1c1c;
}

how i can add a twitter like yours ? it is a plugin?