Yhyjyk

I'd like to know why the "Theorem", "Lemma", etc are not in bold in my text. The code is

documentclass[12pt,a4paper]report
usepackage[utf8]inputenc

usepackage[english,greek]babel
usepackage[square, numbers, comma, sort&compress]natbib % Use the "Natbib" style for the references in the Bibliography
usepackageverbatim % Needed for the "comment" environment to make LaTeX comments

usepackageamsmath,amsfonts,amssymb,amscd,mathrsfs,enumerate
usepackageamsthm
usepackage[all]xy
usepackagerelsize
usepackagepifont
usepackagemathtools
usepackage[all]xy
usepackagerelsize
usepackagefourier
usepackageesvect

usepackagepgf,tikz
usepackagemathrsfs

usepackage[capposition=top]floatrow
renewcommandqedsymboltextcolorblack$blacksquare$
allowdisplaybreaks 

...

usetikzlibraryarrows

DeclareRobustCommanden[1]%
 foreignlanguageenglish#1%


newcommandengbeginotherlanguageenglish
newcommandeengendotherlanguage


theoremstyleplain
newtheoremthmΘεώρημα[section]
theoremstyleplain
newtheoremdfn[thm]Ορισμός
theoremstyleplain
newtheoremprp[thm]Πρόταση
theoremstyleplain
newtheoremlem[thm]Λήμμα
theoremstyledefinition
newtheorempx[thm]Παράδειγμα
theoremstyledefinition
newtheoremcol[thm]Πόρισμα
theoremstyleremark
newtheoremnote[thm]Παρατήρηση
theoremstyleremark
newtheoremprf[thm]Απόδειξη
theoremstyleremark

Thanks in advance!

The fourier fonts don't support Greek, so you get warnings about font substitutions.

An available Greek font that's similar to Utopia is GFS Artemisia.

If you add

usepackagesubstitutefont

substitutefontLGRrmdefaultartemisia

to your preamble, you get, from

beginthm
enTheorem Theorem % just for comparison
$alphabeta$ ab
endthm

the output

Other possible choices are, instead of usepackagefourier,

usepackagetempora
usepackagenewtxmath

which will use a Times-like font. Here substitutefont is not necessary.

Note. It's not necessary to repeat theoremstyle declarations. The scope of one lasts until countermanded by another similar declaration.