You are seeing CSS as your main language because it is the most-used in that repository. Other than adding more source languages to your repository, I do not think there is any manual way to change that displayed language.
Hope this helps!
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To do that you need to:
1. create a .gitattributes file in the root of the project
2. edit that file and add a line to ignore the css language file
*.css linguist-detectable=false *.js linguist-detectable=true
3. add, commit and push to remote and see the result
With this 3 steps you can ignore detection for a specific languages. You can also add more attributes, for example something like this:
*.css linguist-detectable=false *.js linguist-detectable=true *.java linguist-detectable=false
this attributes ignore the java files.
Hope this helps...
Ps: if you need some screenshots ask...
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