If you need to upload a new theme to the Portal, put it in the themes folder (<NEWIRED PORTAL FOLDER>/themes).
Then, both if you uploaded one or modified one, follow the instructions below.
Access the Portal and click on the Addons button.
Scroll down until you find the Themes section, and click on the Reload button you find there to apply your changes.
If your changes have broken the theme, it simply won't show among the available themes and a warning box will pop up.
If you have modified the currently used theme and the changes have broken it, Newired default theme will be used and a “missing theme” indication in the Site Settings of both the Portal and the Editor will appear. If you restore the broken used theme and press Reload, it will be automatically reset as selected theme.