As you can see, this feature would make it obvious which panel is active AND which tab is active. Since it's only a border, it wouldn't interfere with my current background color choices or View Modes settings.
Part of this suggestion is to let the user choose the line thickness and border color. Another option might be to make the border color the same as the active tab color.
I want to point out that I've tried many combinations of ActiveTabColor, ActiveTitle, and ActiveTitleText, none of which I found satisfactory and fiddling around with these settings is very tedious. Of course I can choose a really bright color for ActiveTitle but it's ugly. You may have noticed in the screenshot that I'm an avid user of View Modes for adding icons and colors to tabs. Using a bright color for ActiveTitle makes the active panel more visible but often clashes with my tab colors.
I've been using Total Commander for many years (license number around 19000) and I've been hoping for a feature like this for a long time. In fact, I'm always interested in forum posts asking about it. Here are a few examples going back 15 years:
As you can see, I'm not the only TC user interested in a feature like this.2018-05-14
A setting to improve visibilty of active pane, active tab?
Change background color for active / inactive panes
Active tab - custom text color and font properties
How to know which window is active (left or right)?
8.0 nothing new in GUI
Panes with different background color... possible?
[REQ] Differentiate active / inactive panels more
[REQ] highlight active tab label
feature request-active side inactive side
I hope you'll consider adding this feature to make the active panel and tab so clear and obvious that even a dummy like me can easily see it and easily use it.