Thank you, Oliver, for your quick reply!
This time, I made sure not to delete the custom changed CSS. So I just put the old one back and everything looked fine.
Then I read your reply and tested my form with IE 7. Sure thing, it was not working (i.e., CAPTCHA image not changing, hanging after submit).
It turned out the form was not working with Firefox as well.
I had to change
var sajax_uri = '/'; to var sajax_uri = '/blog/';
in the cforms.js because it had been replaced with the new one.
I also added the followings at the end of the dark-rounded.css as you said in another forum topic:
*+html ol.cf-ol,
*+html ol.cf-ol li,
*+html span.reqtxt,
*+html span.emailreqtxt {
display: inline-block; /*for ie7*/
}
The form finally began to work properly after I clicked on "Update Changes" of the Basic CSS editor and "Update Settings" of the CAPTCHA Image Settings (a few times).
Thank you for your help!!!!!
(I thought these detailed descriptions might help others, but please feel free not to post this one if you feel these would confuse others.)