Hi Nitesh, you have posted your CSS in the correct place. Some themes have a box within that theme's settings where you can add CSS - but I personally use the Additional HTML option in the main, as it saves me having to move the CSS when changing themes.
In terms of updating the mobile app, this is much trickier - I have created a blog post on this https://davefoord.wordpress.com/2023/05/31/moodle-tricks-of-the-trade-adding-your-own-css-to-the-moodle-mobile-app/
But - this only works if you have a premium account, if you don't then you cannot change the CSS at all within the mobile app.
And finally - I know it isn't the purpose of your question, but I would get someone to check the CSS you are adding: From an accessibility perspective - fully justifying text is not a good idea, and where you have set a specific size for the fonts, this would be better using relative sizes e.g. rem rather than px