Yes, this is possible.

Pls do the following:

Go to the file : openerp\addons\web\static\src\css\base.css 

search for the class  ui-tabs-active, and you'll find the following class

.openerp .oe_notebook > li.ui-tabs-active > a {
  color: #4c4c4c;  
}

add to it the font-weight:bold; line to be as follows

.openerp .oe_notebook > li.ui-tabs-active > a {
  color: #4c4c4c;
  font-weight: bold;
}

refresh your browser and you'll get the desired result

Note:

1. I got the class name by using the inspect feature in the browser

2. I'm not a css developer, and this is the first time I try to change some Odoo styles. 

    I gave you the first light, and you have to check with a css developer to know how to do this change without editing the base.css file

I hope this reply is helpful.

=============

I checked again and found this link https://www.odoo.com/nl_NL/forum/help-1/question/how-css-and-js-inheritance-works-on-odoo-v8-55278

pls read it and you can do the changes you wished in a standard way. I have to read it/apply it too.