the setting for the captcha are passed was URL variables in the captcha image tag:
img src="webassist/captcha/wavt_captchasecurityimages.php?field=Security_Code&font=fonts/MOM_T___.TTF" alt="Security Code" class="Captcha" />
there are variables settings available like noise color, which is the color of the background dots, you could change that to white:
img src="webassist/captcha/wavt_captchasecurityimages.php?field=Security_Code&font=fonts/MOM_T___.TTF&noisecolor=FFFFFF" alt="Security Code" class="Captcha" />