I can not read (see) the menu buttons for bold, italic, align left, align right, align center, and others. The buttons are present, but their symbols are invisible, I can only identify them by hovering my mouse pointer over the menu button. This has made it very difficult to use the LibreOffice products. How can I fix this. This issue appeared when I installed Linux Mint 21.3 and installed LibreOffice 7.6.4 and 7.6.5. I did not see it with Linux Mint 20.3 and earlier versions of LibreOffice. Thank you.
Jim McCrary
If you haven’t yet, you might try resetting the user profile?? LibreOffice user profile - The Document Foundation Wiki
Same here: How somebody could have thought that creating white symbols on a bright grey background would be a good idea, is beyond me, so I totally agree with Jim McCrary, where he complains that buttons in Libre Office have become invisible, after version 7.3:
After updating my Linux Mint Cinnamon 21.3 to Linux 22 (“Wilma!!!”) I had the same problem: My buttons in Libre Office had become invisible and I was ready to revert to Linux 21.3 (“Virginia”), but I found the solution, after some searching. The “solution” that Joshua4 proposed did not solve the problem, as it offers all sorts of buttons in all sorts of lay-outs, but all white on bright grey background.
My Libre Office is in Dutch language. I am not totally sure about the names of the buttons in English, but I’m trying to help. (Normally I refuse to register accounts).
Now please open an empty document in Libre Office Writer. Click on “Extra” (the third menu if you start counting from the right). I clicked on “opties”, which should be “Options” in English, I suppose. A screen opens, with some mini triangles that you can click on to open the sub-menus. Click on “Libre Office”. The first from the top is a user profile, but I have no name on the internet; i insist on privacy, so I skipped that. The second word should be “General” in English; Skip that. Click on “Image” and now look at the middle of the screen you are working in. It says “pictogramthema” in Dutch, but I can only guess the English translation might be “thumbnail theme”. Anyway, Jim McCrary and all the others suffering from invisible buttons will want to change that style to “Colibre”, whereas the default setting was “Colibre (SVG+dark)”.
Now look at your buttons for bold, italics and underline: They are black again! 
I hope this helps some people. Feel free to post your “thank you” remarks below. 
Thinking again, this flaw may have come from some young designer who thinks the whole world is as gothic as him/herself and always is working in dark modus. Sure, white symbols on a black background make sense, but I have always left the dark modus for the gaming nerds. I just want to type a document.
That designer is likely a classmate of people who thought that our new car should be a computer on wheels, following our each and every move. I totally hate that! One of those modern cars almost killed me by ghost braking and sending my into aquaplaning on the motorway. Oh, no CD-player either… I miss my 1990’s technology!!! 