There are a number of ways to modify the look and feel of Domoticz, ranging from simple CSS modification, through to complete replacement of the dashboard.
You can find current and popular modification on the Domotiz forum. Some examples are:
Built in themes
You can change Domoticz a little bit through built-in themes, which mostly change the colour scheme. Dark themes are useful in case you have a tablet on your wall as an interface, so that it doesn't give too much light.
You can add Custom icons to the interface. Lots of users have created iconpacks. You can find those in the forum.
You can modify the CSS of Domoticz in two ways. The simple way is to install a browser plugin like Stylebot or Stylish, and let your browser override the style. You can then add the CSS via that plugin.
Another way is to modify a file in your Domoticz installation called custom.css. Whatever you add there will override the default styling. This will change the look for all browers, even those without plugin capability, like mobile phones.
More rigorous modification of the theming system is possible too, where you replace more files.
Some users have created complete alternative interfaces and dashboards that circumvent the default web interface completely.