Too little contrast between focused and backdrop windows

## What Happened
There's too little window titlebar contrast between active and inactive windows.

This is especially visible while using the Light theme, but the Dark theme also has a tad too low of a contrast.

Examples:

**Firefox: _Active_ window:**


**Firefox: _Inactive_ window:**


**Files: 2 windows side by side**

## Expected Behavior
Enough visible contrast to evaluate which window is currently active, if multitasking between windows of the same application. Inactive window font colour could be dimmed, too.

## Steps to Reproduce

1. Open an application, e.g. Files (native) or Firefox (GTK-based)
2. Open a new window from that app
3. Put windows side by side
4. Quickly decide which window is currently active

## Platform Information
Elementary OS 6, Odin - kernel 5.11.0-27-generic
Hardware: ThinkPad-T410s
Processor: Intel® Core™ i5 CPU 540M @ 2.53GHz
Video hardware: Intel® Ironlake Mobile + NVIDIA Corporation GT218M [NVS 3100M] (Optimus)
Video drivers: i915 as primary display, nouveau (headless) as DRI_PRIME backend
Memory: 7,6 GiB