Elgg 3 comes with the theme as part of its core engine and no longer as a separate (bundled) plugin. On Elgg 2 the default theme was implemented in the same way only that the Aalborg theme came as an additional theme plugin that was bundled and was more or less the de-facto default theme due to providing a more modern look and other features.

For customizing the look and feel of Elgg 3 you would have to create a plugin on your own (at least for the moment there are no compatible theme plugins available that you could use as starting point). Most likely you would start by adding some of the core css files (from the core view folder) to your plugin to be able to change the look of the site as these modified css views would then override the Elgg core css files. Some info about creating themes can be found at http://learn.elgg.org/en/3.0/guides/themes.html.

@iionly Thank you. I managed to make a plugin to override default CSS. Now I only need to find the file that holds default variables like $(border-color-soft) to make colours change simpler and I guess that's it.

@ckubs You can find all global CSS variables in engine/theme.php

To add or alter variables, read docs