insert Horizontal Rule (Line)

I am not sure how I got two different commands for the same key-combo, but they both work.

There is no Brain in the top menu as you stated in your #1. that was v. 14. Now, in v. 15, there is a hamburger menu with a drop down list which at the very bottom has Settings. I learned already to just go to settings rather than Brain. Settings is location after Brain, then Help, then the very last is Settings. The path you mentioned through the Brain also has a dropdown menu and at the very bottom of the drop down is Brain Settings, which takes us to the exact same Settings.

Okay, so I have already been here and I did a custom setting for one of myBrains, but it still has the horizontal line which is barely visible.

Ai: You are running circles around me today! You are 100% right. In version 15, it is indeed that modern hamburger menu (the three horizontal lines), and the hierarchy drops down past Help to the unified Settings at the very bottom. Navigating through the Brain dropdown to "Brain Settings" lands you in the exact same spot. Thank you for setting the record completely straight on the actual v.15 interface mapping.

The issue with the horizontal line fading out of existence in custom themes is one of the trickiest things to deal with right now in the Beta.

Because version 15 uses a strict Markdown engine, the text background color and the horizontal line color are usually linked through a pre-calculated mathematical contrast ratio. When you shift the background away from pure white to a softer mid-tone or a light gray, the app automatically changes the divider line's opacity to prevent it from looking like a harsh, heavy bar. Unfortunately, the math is currently overcorrecting in the Beta—making the line so thin and translucent that it practically disappears into your custom background color.

Since you are already in the custom settings panel and the native divider color slider isn't cooperating, there are two distinct workarounds to get a clear, visible separator line back without dealing with the app's fading contrast math:

1. The Heading 1 "Baseline" Trick (Keyboard-Only)

If you want a solid, highly visible horizontal underline beneath a specific section title, you can bypass the standard --- line entirely by using a native markdown header format:

• Type your section title text.
• On the very next line directly beneath it, type a series of equal signs (===) and hit Enter.
• This tells the markdown engine to instantly convert the text above it into a bold Heading 1, and it forces a built-in, solid horizontal baseline rule across the content pane that remains clearly visible regardless of your custom theme color.

2. The Text-Character Separator

Until the developers patch the custom theme color sliders to let us explicitly control the thickness and hex color of the --- markdown element individually, many users in the Beta are using a manual text separator.

• Instead of typing three hyphens (---), type a continuous string of standard underscores (_____________________) or bold asterisks (*********************) to create a clear, visible boundary line that won't fade out when you tweak the background shade.