The Public Knowledge Project has made available an OJS 3.0+ theming guide: https://www.gitbook.com/book/pkp/pkp-theming-guide/details but it assumes proficiency with HTML and CSS.
The PKP team recommends the creation of a child theme in order to make any changes:
“A child theme is a theme that extends an existing theme by adding or modifying the styles, scripts or templates loaded by that theme. A child theme is the best way to make small modifications to an existing theme, without editing that theme’s files. This allows you to take advantage of updates to the parent theme, keep your custom code separate, and minimize conflicts when upgrading the software.” https://github.com/NateWr/default-child
For those looking to do some basic tweaking, there are a number of elements that can be tweaked via the dashboard for the standard OJS 3.0+ installation. No HTML or CSS knowledge is needed. See here: https://pkp.gitbooks.io/ojs3/content/en/website_settings.html For many, these options are enough to reach a desired result.
For those with particular design in mind, the YDJ team can try to approximate your design in OJS 3.0+ by creating a child theme as specified above. The YDJ team can work from your mockup and translate elements of the design into OJS 3.0+. Note that there are no designers on the YDJ team. Editors will need to provide a mockup or share specifically what they would like to achieve, i.e. background to be dark blue.
Have a look below at the existing OJS 3.0 journals currently hosted as it will give provide an idea of what is possible:
https://pai.journals.yorku.ca
http://csri.journals.yorku.ca
https://core.journals.yorku.ca
http://justlabour.journals.yorku.ca
https://litte.journals.yorku.ca
https://claudel.journals.yorku.ca
https://soi.journals.yorku.ca
https://tessera.journals.yorku.ca
https://lobo.journals.yorku.ca/
The following two themes are being tested on the library sandbox site:
https://sandbox.journals.yorku.ca/index.php/redtheme
https://sandbox.journals.yorku.ca/index.php/bluetheme
https://sandbox.journals.yorku.ca/index.php/manuscripttheme