Power Apps allows you to create theme that you can apply to all modern controls to keep consistency of theme
First, you have to create app using modern control. To enable MC:
- On the command bar, select Settings > General.
- From the Preview tab, set the toggle for Modern controls and themes to On.

Then you can create the theme to match your company's branding here:

------------- I don't know why I can't reply to this topic anymore, so I will update the answer here @AsherNg1990
The modern control differs significantly from its original counterpart. They were designed to replace the old one, but some are still in the development phase. If the modern control lacks a property you need, there is little we can do but find a workaround solution.
For instance, the modern toggle control now features a Checked property, which you may already be familiar with. As a result, you can make minor adjustments to your code to adapt to this change. Some controls, however, are missing the OnSelect property, and you will have to wait for an update. For example, the modern table control only recently gained the OnSelect option.
In terms of theme consistency, I believe the modern control is the best and most convenient option
I have another approach but cost much more time, that you can declare theme setting in app's OnStart property.
Set(varSysColor, RGBA(234,112,201,1));
Set(varSysFontSize, 12);
and apply that variable to each control
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