It is always a good idea to keep your custom CSS separate from the existing template or component styling. There are 2 ways to do CSS override; either through EasyBlog custom CSS or template custom CSS.
If you're using built-in EasyBlog custom css editor, you may navigate to Components > EasyBlog > Themes > Custom CSS and edit your CSS there. You may also open up the physical file in your current Joomla template: /templates/<yourtemplate>/html/com_easyblog/styles/custom.css.
However, if you're utilizing template custom css, most times you may find it under /templates/<yourtemplate>/css/. Please consult your template provider if your template's custom css file is not located here.
Deprecated selector #fd.eb
Since EasyBlog version 5.1.x, we have deprecated the usage of #fd.eb by replacing it with #eb. Hence, if you're using #fd.eb in your custom CSS, your styling will not be working. To fix this, you will need to do search & replace for all #fd.eb selectors in your custom CSS and replace it with #eb.
You will need to make sure that the class selectors are defined correctly according to the new EasyBlog DOM.
Example:
/* This is your old CSS override */
div#fd.eb.eb-wireframe .eb-header .eb-navbar {
background: red;
}
/* This is your new CSS override. */
/* You will need to make sure the class selectors are correct. */
div#eb.eb-wireframe .eb-header .eb-navbar .eb-navbar__body {
background-color: red;
}