html
css
.logo-img path {
If your goal is just to change the color of the logo, and you don't necessarily NEED to use CSS, then don't use javascript or jquery as was suggested by some previous answers.
To precisely answer the original question, just:
Open your logo.svg
in a text editor.
look for fill: #fff
and replace it with fill: #000
For example, your logo.svg
might look like this when opened in a text editor:
... just change the fill and save.