Error messages when using sassc to compile gtk.sass to gtk.css

Command I use is **sassc gtk.sass gtk.css** from the ~/.themes/Dracula/gtk-3.20 folder. It also makes the theme take a long time to load when choosing it with gnome-tweaks.

Here is the output:

` ⊙ sx@ares ⊙ /home/sx/.themes/dracula-custom/gtk-3.20 ⊙ sassc gtk.scss gtk.css

WARNING on line 125, column 25 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_base-states.scss:
Compound selectors may no longer be extended.
Consider `@extend %selected_items, :disabled` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 133, column 25 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_base-states.scss:
Compound selectors may no longer be extended.
Consider `@extend %selected_items, :backdrop` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 13, column 28 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked, :last-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 14, column 28 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked, :first-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 21, column 28 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %button_basic, :drop(active)` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 34, column 78 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked, :first-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 35, column 77 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked, :last-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 36, column 76 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked, :only-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 39, column 72 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked_vertical, :first-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 40, column 71 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked_vertical, :last-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 41, column 71 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_comboboxes.scss:
Compound selectors may no longer be extended.
Consider `@extend %linked_vertical, :only-child` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 159, column 30 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_header-bars.scss:
Compound selectors may no longer be extended.
Consider `@extend *, :link, :selected` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 36, column 11 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_links.scss:
Compound selectors may no longer be extended.
Consider `@extend *, :link` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 126, column 15 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_notebooks.scss:
Compound selectors may no longer be extended.
Consider `@extend %button_basic, .flat` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 44, column 25 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_popovers.scss:
Compound selectors may no longer be extended.
Consider `@extend modelbutton, .flat` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 31, column 31 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_progress-bars.scss:
Compound selectors may no longer be extended.
Consider `@extend %scale_trough, :backdrop` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 37, column 33 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_progress-bars.scss:
Compound selectors may no longer be extended.
Consider `@extend %scale_highlight, :backdrop` instead.
See http://bit.ly/ExtendCompound for details.

WARNING on line 103, column 13 of /home/sx/.themes/dracula-custom/gtk-3.20/widgets/_windows.scss:
Compound selectors may no longer be extended.
Consider `@extend %button, .flat, .suggested-action` instead.
See http://bit.ly/ExtendCompound for details.
`

Start by reproducing the reported `sassc gtk.scss gtk.css` command in the `gtk-3.20` theme directory and inspect the warnings in the named `widgets/*.scss` files. Compare compilation and theme-loading behavior with the reported output; the issue does not define a specific target for resolving the warnings or measuring completion.

Written by the indexing model from the issue text.