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There is any extension available for Filters?
Can you explain what kind of functionality you're looking for?
I want change the Colour of Filter, Colour of Font.
to change averall themes of Qlik app you can create your own theme. You can use https://sensetheme.com/edit
If you mean that you want to change "Green, Gray and White" colour of selections in filters I am worried that you are not able to do so.
Some extensions are e.g.
Qlik Sense Option List - Qlik Branch
SenseUI-Filter - Qlik Branch