Hi,

Indeed, you have to make override, becuase in that template forms by default were on dark background, use custom.css file or Custom CSS field.

For example, You should to use those lines:

#donation_form input::placeholder, 
#donation_form select::placeholder,
#donation_form textarea::placeholder, 
#donation_form input[type="text"]::placeholder, 
#donation_form input[type="password"]::placeholder, 
#donation_form input[type="datetime"]::placeholder, 
#donation_form input[type="datetime-local"]::placeholder, 
#donation_form input[type="date"]::placeholder, 
#donation_form input[type="month"]::placeholder, 
#donation_form input[type="time"]::placeholder,  
#donation_form input[type="number"]::placeholder, 
#donation_form input[type="email"]::placeholder,
#donation_form input[type="url"]::placeholder, 
#donation_form input[type="search"]::placeholder, 
#donation_form input[type="tel"]::placeholder, 
#donation_form .uneditable-input::placeholder {color: #333 !important; }

#donation_form input[type="number"],  #donation_form input[type="email"], 
#donation_form input[type="url"], #donation_form input[type="search"], 
#donation_form input[type="tel"], #donation_form select, input#amount,
#donation_form textarea,    #donation_form input[type="text"] 
{border-color: #aaa !important; outline:1px solid #ddd; 
color: #333 !important; border-radius: 4px !important;}

#donation_form input[type="text"]:hover {border: 1px solid #000 !important;}
.donation-default-switch-amounts label.otheramountlabel {outline: 1px solid #aaa;  }

Please do not touch template.css file (!) we informed about that in Helix documenation.

Check if JoomDonation has Custom CSS field in Settings/Options, if yes, use it. If not, add inside Custom CSS in Template Options.