Use this code as the Custom CSS:

{xmt_id}.xmt ul li.tweet_list{color:#000;}
{xmt_id}.xmt a{color:#000;}

{xmt_id}.xmt ul li.tweet_list{color:#000;} is to change the text color.
{xmt_id}.xmt a{color:#000;} is to change the hyperlink color.

Thank you...

. . . as it is displayed using short-code on a page (not in sidebar) but nothing in css.css seems to facilitate changing font-size.

Where do I have to go to make the edit?

Here is the page we are talking about: http://dowestillhavethewilltolive.com/earthslife/

Use this code as the Custom CSS:

{xmt_id}.xmt ul li.tweet_list, {xmt_id}.xmt a{font-size:13px !important}

I did change the font size as recommended but it affected my username too.

What username? The widget title?
If so, add this code too:

{xmt_id}.xmt a.header_48{font-size:18px !important}