Imagine having a nice PhpBB3 forum where you'd love to attach inline photos in your posts. The problem is that max height for the image size is set to 350 by default for the prosilver theme. I managed to change it only in the following way:

Open your forums mysql database and find the table phpbb_styles_theme. There go to row theme_name=prosilver, find theme_data field. There will be your whole css for the theme loaded. Find:

.attach-image {
margin: 3px 0;
width: 100%;
max-height: 350px;
overflow: auto;
}

Change it to:

.attach-image {
margin: 3px 0;
width: 100%;
max-height: 1350px;
overflow: auto;
}

There are two things to consider:

1. The above is probably loaded at some time from the file styles/prosilver/theme/content.css. It is a good idea to make the same change there too.

2. Make sure to clear your forum's cache, i.e. delete everything inside the cache/ directory.