Hi,
this is due to your template, you can fix it via CSS. We can help you but we need to see your site, if you want then you can use the form below the reply in "Site Details" tab to send me your site link (no needed access, I only need to see frontend problem).

site details below

which file I have to add the code in?

Hi,
try to create file /templates/your-template/css/custom.css and put the code in this new file

or

many template provider allow to add CSS code as parameters into Joomla Backend, you should check this into Administration->Templates->Choose your template

I added the line in bootstrap.css but the .dropdown-menu entry from bootstrap,css always overwrites this

.jsndatepicker.dropdown-menu {
background: white;
}
.dropdown-menu {
position: absolute;
top: 100%;
left: 0;
z-index: 1000;
display: none;
float: left;
min-width: 160px;
padding: 5px 0;
margin: 2px 0 0;
list-style: none;
font-size: 14px;
text-align: left;
background-color: #444444;
border: 1px solid #444444;
border-radius: 0;
-webkit-box-shadow: 0 6px 12px rgba(0, 0, 0, 0.175);
box-shadow: 0 6px 12px rgba(0, 0, 0, 0.175);
background-clip: padding-box;
}

even .jsndatepicker .dropdown-menu {
background: white;
} didnt helped

Hi,
I have tried with a console and works, only you need to put after bootstrap.css.

Have you followed my instructions?

try to create file /templates/your-template/css/custom.css and put the code in this new file

or

many template provider allow to add CSS code as parameters into Joomla Backend, you should check this into Administration->Templates->Choose your template

if Yes try to clean cache.
Otherwise enable me backend access. we will check this.

Hi,
found the problem, you copied the code wrong.

you wrote ".jsndatepicker(space).dropdown-menu" correct is without space

Fixed on your site