>> Altought I don't understand why change of colours have such an impact
That question is not quite really simple to answer.
A forum like the jref consists of two main parts: the database(s) and presentation part. The latter is also known as "skin", GUI (graphical user interface) or template (there may be some other names for this).
This separation has a big advantage: You can change the look, the visual structure and the charcter set without changing the data.
Programming of the skin generally does not only consist changing of colors, but also the setup of tables, design of buttons and so on. Changing colors means although the need of changing character sets - you won't see blue characters very well on a blue background..... ;-)
The need for using the proper character settings for japanese and taking care of it while programming the layout imho isn't simple task at all.
I'm sure that setting this up correctly is some kind of a difficult task that I won't like to do .....

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