body {
background-color: rgb(150, 150, 150) !important;
}
.container table tr td {
background-color: rgba(0, 0, 0, 0.7) !important;
}
td div, td p {
color: #F2F2F2 !important;
}
.header {
background-color: #000000 !important;
}
#pages, #upbutton, #toggle {
color: #F2F2F2 !important;
}
.pages:hover, .active, #upbutton:hover {
background-color: #505050 !important;
}
You can also use developer tools (Ctrl+Shift+I) for searching classes/id of elements. For better color you can search "html color" or use this page https://www.w3schools.com/colors/colors_picker.asp
Edited: I see what you solved issue before when I printed answer:)

Anyone can fork this addon from this site :) I don't upload because this is addon created for themselves (with same restrictions) and have really things in code what cause a shame (but it work as expected and I don't change this); maybe one day it happened.

You can just use Ctrl+Left Mouse Click on folders for open all in tabs. I don't think what this is would be good change because it may lead to accidentally opening all bookmarks.

It's two point why this is impossible.
First: I don't think what this is good idea, because it absolutelly another GUI.
Second (and mostly important): it impossible because FF for Android doesn't support bookmarks API (take a look table "Browser compatibility" https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/bookmarks )

I added option for disabling context menu on about:addons page in 0.1.30.
About subreddit... I do not use social networks like reddit, twitters, etc and I not very communicative. If you wish you may create this ourselves and I'd add hyperlink to description as unofficial users forum.

I use cropping images bottom because browser.tabs.captureVisibleTab don't have fields for determinating proportions of creating images. I added your userstyle to description.

Webextensions is not provide direct access from filesystem.
And this is only one loading logic:
Left click: reload from site
Right click: submenu for alternate one time loading methods (or for removing)
On this moment saving structure:
{bookmarkid: dialimage}.
Maybe in future I extend this to:
{bookmarkid: {dialimage: dialimage, dialimageorigin: fromurl}} but it changed only "reload all dials in dialgroup" logic.
But it is probably in future, because it is very rare pattern for me (I created this ext after dead of Speed Dial by Josep del Rio basically for yourself because some different Dials ([]) seems to me as malware).

in the end of rule before ';' like
.container table tr td {
background-color: rgba(243, 0, 0, 1) !important;
}

I do not think that this is possible inside webextension.
But you can do it yourself.
If you use Firefox <57 just copy url from URLbar and paste on about:preferences page as home page.
If you use 57:
1. Open about:debugging page and copy UUID of "Bookmarked Speed Dial"
2. Open about:preferences page and paste as home page
moz-extension://your-UUID/data/index.html

It will be fixed in 0.1.22
Temporary solution:
.imgcontainer {
display: block !important;
}