how to change svg fill color when used as base-64 background image data?

Question 1

I want to post an answer to my own question. No exactly an answer, since it doesn't imply svg as background and base64, but it's my current alternative way, much more effective if you just need mono-color icons: just use svg as css mask, and change background color. Then you can also apply transitions on mouse events. I mean something like this:

mask-image: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" xml:space="preserve"><path d="M11.432 9.512a.636.636 0 0 1 0 .918L2.12 19.742a.636.636 0 0 1-.458.201.637.637 0 0 1-.46-.201l-1-.998C.068 18.609 0 18.459 0 18.285s.068-.329.199-.461l7.854-7.852L.199 2.118A.638.638 0 0 1 0 1.66c0-.174.068-.327.199-.46l1-.998A.634.634 0 0 1 1.66 0c.172 0 .325.068.458.201l9.314 9.311z"/></svg>');
mask-size: auto 12px;
mask-repeat: no-repeat;
transition: background-color 200ms;
background-color: #ff0000;

Please see this fiddle for come more code and demo:

https://jsfiddle.net/o25beLqj/

Also note a couple of things:

mask-image does not have base64 code inside. It's just preceeded by data:image/svg+xml;utf8,
mask css properties are quite about the same of css background properties, take a look to docs, and you'll see you can do really a lot and they can substitute background images in a variety of use cases

Question 2

Use filter property of CSS. For me I wanted to change the color of the icon to white on Hover:

filter: grayscale(1) brightness(2);

Question 3

Or if you want to do it dynamically try :

var green = '3CB54A';
var red = 'ED1F24';
var svg = '<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"  width="320px" height="100px" viewBox="0 0 320 100" enable-background="new 0 0 320 100" xml:space="preserve"> <polygon class="mystar" fill="#'+green+'" points="134.973,14.204 143.295,31.066 161.903,33.77 148.438,46.896 151.617,65.43 134.973,56.679 118.329,65.43 121.507,46.896 108.042,33.77 126.65,31.066 "/><circle class="mycircle" fill="#'+red+'" cx="202.028" cy="58.342" r="12.26"/></svg>';      
var encoded = window.btoa(svg);
document.body.style.background = "url(data:image/svg+xml;base64,"+encoded+")";

Fiddle Here

Question 4

Base64 for that would be:

url('data:image/svg+xml;base64,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');

using a tool like http://www.base64encode.org/

This doesn't answer your question directly, but it does let us do the following. We can now test to see if:

.icon:hover .style0 {
    fill: red;
}

will work, or use

.icon {
    background-image: url('data:image/svg+xml;base64,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');
    height: 40px;
    width: 40px;
}

.icon:hover {
    background-image: url('data:image/svg+xml;base64,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');
}

which seems inefficient to me because we are forced to replicate a lot of the same information for the hover when all we want to change is the color.

working example