If you use the Fiori Launchpad (FLP) you have 2 options.
- [Most common & recommended] Configure your tile to open the app calling the Component.js file
- Configuring the tile to open the index.html file (UI5 standalone configutarion)
In option 1, the index.html file of your app is not used. It's the FLP the one who plays the role of index.html file, and the app is loaded into the ComponentContainer of the FLP using JS-based navigation (hash navigation). So, in a default app config, the theme you use in your FLP will be the one used in your app.
Besides you can configure which are the supported themes for your app in the descriptor file, setting the property
"supportedThemes": [
"sap_bluecrystal"
]
More info in this link
In option 2, the app will be opened in a new tab, calling the index.html file, so it will use the theme you configure in the index.html file.
In the most common & easy way, you apply your custom themes in the whole FLP and you configure your apps to use it.
I think you should start reading about Theme Designer before you start coding CSS
Furthermore if you want to add just a few CSS classes, read this