Just noticed your post, nice site
I assume you are have cloned the template, or your theme comes with its own template. Blocks are handled by XOOPS templates, and if you look at the relevant template in system-admin templates you will find and identify any relevant calls to the classes in the style.css in your themes folder. See XOOPS for dummies and the themes section for cloning advice, you can't edit the default template.
Qu Number 1
system admin-blocks, look for the "login"" block, click "edit" to the right. and look for "edit template". Near the bottom you will see:
<{$block.lang_registernow}>change it to something like this:
<{$block.lang_registernow}>I will look into the other questions and be back later
QU 2 language/english/user.php
QU 3 fonts for the entire site are defined usually at the top of the style.css like this:
body {background-color: #fcfcfc; color: #000000; font-weight: normal; font-size: 12px; font-family: "Trebuchet MS",Verdana, Arial, Helvetica, sans-serif; margin: 0px; padding: 0px;}
Individual local font definitions in blocks etc overide this setting. Remember that the definitions for the links in blocks are hyperlinks so look for this kind that start with an "a":
td#mainmenu a {background-color: #E0E0E0; margin: 0; border-top: 1px solid #F2F1F2; border-bottom: 1px solid #ADACAD; padding: 0;}
td#mainmenu a:hover {background-color: #D1D0D1;text-decoration: none;}
you can add a color definition like this if it does not exist.
color: #FF0000;
Some themes use images for the backgrounds colors for the titles, these are often defined in the style sheet as a url link to the image, and you may in this instance need to change the image color itself before you see any changes.
I have not looked at zaja system yet, I hope this points you in the right direction anyway.