Scrollbars that jump many pages when you click the wrong place

GTK themes can contain general GTK settings. Devuan Excalibur's theme contains a settings.ini containing the line

gtk-primary-button-warps-slider = false

This makes the scrollbar only scroll by one page if you left-click it somewhere other than the handle.
If you want this behavior all the time and not just when you're using Excalibur's default theme, simply create ~/.config/gtk-3.0/settings.ini and add the same line to it.

Thanks for posting this!  The same setting works on Daedalus also.  Note that if you're creating a settings.ini file, that code should be in a "[Settings]" group, like so:

[Settings]
gtk-primary-button-warps-slider = false

Otherwise you'll get a gtk warning, failed to parse settings.ini: Key file does not start with a group.

A lot of people use custom setups or respins, however, so they might not see the same defaults. Based on a conversation today with fsmithred, I thought the following on how such things are set might be helpful:

Scrollbars that vanish when your mouse isn't moving over the right window

GTK calls these "overlay scrollbars", and their intended benefit is saving screen real-estate for people who only scroll with a scroll-wheel or track-pad gesture anyway, but they force you to take actual action to see where you are in list or document rather than having an always-visible slider, and if you're the type that actually does click on scrollbars, they are really annoying.

This is solved by default in Devuan Excalibur because a package to change the default behavior is recommended by the default theme, but if you don't have it simply

apt install gtk3-nooverlayscrollbar

What this package actually does is introduce the environment variables

GTK_OVERLAY_SCROLLING=0
LIBOVERLAY_SCROLLBAR=0

into your X session, so you can also set those yourself or add them to a custom Wayland session.

Scrollbars that jump many pages when you click the wrong place

GTK themes can contain general GTK settings. Devuan Excalibur's theme contains a settings.ini containing the line

gtk-primary-button-warps-slider = false

This makes the scrollbar only scroll by one page if you left-click it somewhere other than the handle.
If you want this behavior all the time and not just when you're using Excalibur's default theme, simply create ~/.config/gtk-3.0/settings.ini and add the same line to it.

Titlebars that don't follow theming and have random widgets in the way of clicking and dragging

This is called "client-side decoration". It's what web browsers use to display tabs at the very top of the screen where you can just bounce your mouse off the wall and grab them (assuming no top panel), but they're also being used these days in a wild number of places where they provide no benefit unless you consider breaking theming a benefit.

This is already solved by default in Devuan Excalibur because a package to change the behavior is recommended by the default theme, but if you don't have it, simply

apt install gtk3-nocsd

This should automatically configure things to not draw their own ugly titlebars... unless they're GTK4 GNOME apps where that is a core feature. (For a detailed explanation of how this actually works read /usr/share/doc/gtk3-nocsd/README.Debian)

If you've set up a custom session (eg. Xfwl4) where the behavior still isn't right simply

export GTK_CSD=0

And if you think a particular application, for example Firefox, is responsible enough to draw a custom titlebar in a useful way, you can override the behavior for a single application by starting it like this:

GTK_CSD=1 firefox

or in a .desktop file:

sh -c "GTK_CSD=1 firefox"

Toolbar/sidebar icons that are colorless black outlines

Correction: Excalibur's default theme fixes this too.

GTK3 recognizes two separate classes of icons "regular" and "symbolic". The idea is that even at the same icon size there's a difference between a recognizable shape that represents something, like you might see in a toolbar, and an actual full pictograph of the same thing, like you might see in your file manager. A lot of icon themes, including Excalibur's default one, do not provide symbolic icons, so when programs request them, the system pulls default versions from Adwaita that look like the black line art on highway warning signs.

If that's your cup of tea, great, but if you prefer being able to tell items apart by color as well as shape and don't want to set up a different icon theme, it's possible to edit your favorite GTK theme to use CSS that sets everything to "regular" the way it used to be. Devuan Excalibur's default theme, for example, does this. Just put the following in ~/.themes/YourFavoriteTheme/gtk-3.0/gtk.css:

* { -gtk-icon-style: regular }

This is, of course, specifically for GTK applications, and will not affect Qt applications like Krita that are also fond of toolbars that distinguish things by shape alone and not color.