too much specific question on quite wider subject... but, here is answer:

option 1: Change theme in odoo website to one of existing (go to: "Customize/Change Theme" from website), or create/add your own theme...

option 2: through custom module (say named as my_module):

__openerp__.py:

...
    'depends': ['website'],
    'data':['my_module_view.xml'],
...

my_module_view.xml:

<openerp>
    <data>

        <template id="assets_frontend" name="my_module assets" inherit_id="website.assets_frontend" >
            <xpath expr="." position="inside">
                <link rel='stylesheet' href="/my_module/static/src/css/style.css"/>
            </xpath>
        </template>


    </data>
</openerp>

static/src/css/style.css:

a {
    color: #CA4040;
    text-decoration: none;
}

option 3: it may work to change "/web/static/lib/bootstrap/css/bootstrap.css" or "/website/static/src/css/website.css" or whatever is original location of a {} style in base code, but that's not a recommended way, as any further update of web/website modules will discard your changes...

I suggest you to stick with first,  or second option.