Update CSS part

[Nilvus]

This fix #414

@jade-nl, @elstoc and @difrkaguilar: this improve CSS part as requested on #414 and follow so links provided by @jade-nl in that issue.

Those links are old now, so even if I read some parts to work that, it's possible that I miss something. So your feedback are more than welcomed to be sure this will correctly improve that part.

And of course, as my english is not my first language, I'm quite sure english is not the best one!

[jade-nl]

Just had a look at the proposed changes; This looks a lot better then before.

I do see some things that could probably be phrased a little better, but, like you, I'm not a native English speaker so maybe @elstoc can fix/change those.

I'm not sure about the part that describes that you could create your own CSS but shouldn't, this part:

You could create your own CSS file but this will need to choose which theme to import and set it's path hardcoded in your CSS file depending on your system also. This is not the best way. darktable provide a better and simpler way. darktable provide a better and simpler way: a text box is provided for this purpose [...]

Keep it terse and don't mention that part. Maybe something like this instead of the above quoted part:

darktable provides a textbox for this purpose [...]

Just my 2c, as always.

[Nilvus]

I hesitate to add that part. I add it because I read some comment about that on links you posted. But was also wanted to not precise that. I here decide to follow comments I read but I agree with your answer so last commit remove that following your proposal.

I also update other parts. That should be, I hope, better english.

[Nilvus]

Last commit should be better. @elstoc: I hope this (and my comments) would help you to improve my text with better english.

[jade-nl]

Looks a lot better now. Just 2 things I noticed:

1. darktable uses America-English so Colourful should be Colorful
2. This sentence doesn't seem to be correct/clear:

Now we have darktable classes first used for main items [...]

Not sure how to improve it, but the previous version, even though it is somewhat bloated, is clearer to me. I'm sure that @elstoc can knead this into something that is clear to all of us.

[difrkaguilar]

And of course, as my english is not my first language, I'm quite sure english is not the best one!

English is not my native language either. But I'm always trying to do my best... and using DeepL too jjjjj

[elstoc]

Rather than merge this and then push copy-edits afterwards, I have created a separate PR (#471) with some additional wording. I hope I have included the key points from this PR but please feel free to review.